UC-NRLF 


ELECTRO-TECHNICAL 
SERIES 


0 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

Microsoft  Corporation 


http://www.archive.org/details/electricityineleOOhousrich 


BY  THE  SAME  AUTHORS 

Elementary  Electro -Technical  Series 

COMPRISING 

Alternating  Electric  Currents. 
Electric  Heating. 

Electromagnetism. 

Electricity  in  Electro-Therapeutics. 
Electric  Arc  Lighting. 
Electric  Incandescent  Lighting. 
Electric  Motors. 

Electric  Street  Railways. 
Electric  Telephony. 

Electric  Telegraphy. 

Cloth,        Price  per  Volume,        $1.00. 


Electro-Dynamic  Machinery. 
Cloth,  $2.50. 


THE  W.  J.  JOHNSTON  COMPANY 

253  Broadway,  New  York 


ELEMENTARY  ELECTRO-TECHNICAL  SERIES 

ELECTRICITY 

IN 

ELECTRO-THERAPEUTICS 


BY 

EDWIN  J.  HOUSTON,  Ph.  D. 

AND 

A.  E.  KENNELLY,  Sc.  D. 


new  Tea^^Lll!_i.^.^i 

THE  W.  J.  JOHNSTON  COMPANY 

253  Broadway 

1896 


Copyright,  1896,  by 
THE  W.  J.  JOHNSTON  COMPANY. 


CONTENTS. 


R/n<n\ 


CHAPTER 

I.  Introductory, 

II.  Electromotive  Force, 

III.  Electric  Resistance, 

IV.  Electric  Current, 


V.     Varieties  of  Electromotive  Force,  107 


VI.     Electric  Work  and  Activity, 
VII.     Frictional    and    Influence    Ma 

chines,         .... 
VIII.     Magnetism,       .        . 
IX.     Induction  of  E.  M.  F.  by  Magnetic 
Flux,  .... 


X.     The  Medical  Induction  Coil, 

iii 


1 
13 
63 

80 


124 

138 
184 

221 

248 


IV  CONTENTS. 

CHAPTER  PAGE 

XL     Dynamos,    Motors     and     Trans- 
formers,        299 

XII.     High  Frequency  Discharges,        .  329 

XIII.  Electrolysis  and  Cataphoresis,  356 

XIV.  Dangers  in  the  Therapeutic  Use 

of  Electricity,  .        .        .365 

Index, 373 


PREFACE 


This  little  book,  entitled  Electricity 
in  Electro-Therapeutics,  is  intended  to 
meet  a  growing  demand  which  exists  not 
only  on  the  part  of  general  medical  practi- 
tioners, but  also  on  that  of  the  general 
public,  for  reliable  information  respecting 
such  matters  in  the  physics  of  electricity 
applied  to  Electro-Therapeutics,  as  can  be 
readily  understood  by  those  not  specially 
trained  in  electro-technics. 

Electricity  has  recently  made  such  rapid 
strides  in  application  to  both  surgical  and 
medical  practice,  that  recent  information, 
concerning  electrical  developments  in  ap- 


VI  PREFACE. 

paratus  and  in  theory,  is  much  in  request 
by  those  interested  in  the  healing  art. 

The  method  of  treatment  adopted 
throughout  the  book  in  the  description  of 
electro-technics  has  been  the  circuital 
method  ;  that  is  to  say,  all  the  phenomena 
of  electricity  and  magnetism  have  been 
considered  as  pertaining  either  to  the 
electro-static,  the  electric,  or  the  magnetic 
circuit,  and  the  laws  of  these  three  circuits 
have  been  developed  upon  analogous  lines. 
The  authors  believe  that  this  treatment  is 
the  key-note  to  a  clear  comprehension  of 
the  numerous  and  often  complex  electro- 
magnetic phenomena  met  with  in  the 
application  of  electricity  to  electro-thera- 
peutics. 

In  thus  aiding  the  general  public  to  read- 
ily comprehend  the  principles  underlying 
the  physics  of  electro-therapeutics,  the 
authors  trust  that  they  are  aiding  the  gen- 


PREFACE.  Vll 

eral  cause  of  humanity  in  enabling  electricity 
to  be  employed  more  intelligently  in  the 
healing  art,  as  well  as  permitting  truth  to 
be  discerned  from  fraud. 

The  authors  therefore  present  this  book 
to  the  public  in  the  hope  that  it  may 
prove  serviceable. 


ELECTRICITY  IN  ELECTRO- 
THERAPEUTICS. 


CHAPTER  I. 

INTRODUCTORY. 

It  has  not  infrequently  happened  in  the 
history  of  scientific  discovery,  that  from 
various  causes,  the  discoverer  has  obtained 
so  incomplete  a  view  of  the  new  fact,  as  to 
entirely  lose  sight  of  its  true  significance, 
and  to  regard  it  merely  from  the  stand- 
point of  some  of  its  unimportant  char- 
acteristics. This  was  the  case,  in  the  cele- 
brated discovery  made  by  Luigi  Galvani, 
in  1786,  concerning  the  existence  of  what 
he  at  first  believed  to  be  the  vital  fluid,  or 


2  ELECTRICITY  IN 

essence  of  animal  vitality,  but  which  was 
afterwards  proved  by  Volta  to  be  essen- 
tially a  new  method  of  producing  elec- 
tricity by  chemical  action. 

Galvani  discovered  that  if  the  hind  legs 
of  a  recently  killed  frog  be  deprived  of 


Fig.  1.— Galvanoscopic  Frog. 

their  integument,  and  the  lumbar  nerves, 
suitably  exposed  in  their  position  on  either 
side  of  the  vertebral  column,  be  connected 
with  the  crural  muscles  by  a  metallic  strip, 
as  shown  in  Fig.  1,  that  these  muscles 
will  be  brought  into  a  spasmodic  activity 


ELECTRO-THERAPEUTICS.  6 

closely  resembling  their  action  during  life. 
It  has  been  alleged  that  this  classic  experi- 
ment of  Galvani  was  the  result  of  chance ; 
that  he  had  prepared  some  frogs'  legs  for 
supper,  and,  happening  to  hang  them 
against  an  iron  balcony,  he  noticed  that 
the  muscles  twitched  as  soon  as  they 
touched  the  iron;  that  is,  went  convul- 
sively through  their  motions  as  in  life,  and 
that  these  motions  were  repeated  when- 
ever the  contact  was  renewed.  Moreover, 
the  power  of  producing  these  convulsive 
movements  was  retained  by  the  limbs  for 
an  hour  or  more  after  removal  from  the 
body.  This  account  would  seem  improb- 
able, since  Galvani  was  well  aware  of 
the  fact  that  an  electric  discharge,  sent 
through  the  legs  of  a  recently  killed  frog, 
would  produce  convulsive  movements  in 
them;  and,  indeed,  he  was  in  the  habit  of 
employing  such  legs  as  a  form  of  sensitive 


4  ELECTRICITY   Iltf 

galvanoscope,  though,  of  course,  Gal  van  i 
did  not  know  it  under  this  name,  but  he 
did  know  that  it  formed  a  much  more 
sensitive  apparatus  for  detecting  an  electric 
current  than  the  pith-ball  electroscope  em- 
ployed in  those  early  days,  as  almost  the 
only  available  means  for  detecting  an 
electric  charge. 

Unfortunately,  Galvani  failed  to  rec- 
ognize the  extreme  importance  of  his  dis- 
covery. Working,  as  he  had  been  for  a 
long  time,  with  the  hope  of  discovering  the 
seat  of  animal  vitality,  he  was  only  too 
willing  to  find  in  this  observation  the 
principle  of  that  vital  fluid  for  which  he 
so  long  and  ardently  had  sought.  He  was, 
therefore,  handicapped  in  the  search,  and 
unfitted,  to  a  certain  extent,  to  weigh 
calmly  the  evidence  presented.  Though 
thoroughly   familiar   with   the   convulsive 


ELECTRO-THERAPEUTICS.  O 

twitchings  produced  by  the  passage  of  the 
electric  discharge  through  the  frog's  legs, 
it  never  seemed  to  occur  to  hi  in  that  what 
he  had  in  reality  discovered,  was  an  en- 
tirely new  method  of  producing  electric 
discharges.  He  only  saw  in  this  observation 
what  he  so  ardently  desired  to  see  ;  namely, 
convulsive  muscular  movements,  due  to  a 
vital  fluid,  which,  he  believed,  came  from 
the  nerve  of  the  animal,  and  was  conveyed 
through  the  metallic  conductor  to  the 
muscles,  where  it  produced  the  char- 
acteristic twitchings. 

The  announcement  by  Galvani  of  his 
discovery,  produced  the  most  intense 
excitement  throughout  the  scientific  world, 
and  his  views  as  to  the  cause  of  the 
phenomenon  were  at  first  generally 
accepted.  Among,  perhaps,  the  most 
ardent      of     his      early      followers     was 


b  ELECTRICITY   IN 

Alessandro  Volta,  who  at  once  repeated 
Galvani's  experiments,  and  began  a  series 
of  extended  researches  on  the  phenomena. 
Volta  soon  reached  the  conclusion  that  the 
twitchings  of  the  legs  of  Galvani's  frogs 
were  to  be  ascribed,  not  to  a  vital  fluid, 
but  to  the  presence  of  an  electric  dis- 
charge, and  that,  consequently,  sight  had 
been  lost  of  the  most  important  part  of 
Galvani's  experiment ;  namely,  that  it  fur- 
nished a  new  method  of  producing  elec- 
tricity. 

Volta  showed,  among  other  things,  that 
the  convulsive  movements  were  more  pro- 
nounced when  the  nerves  were  connected 
with  the  muscles  by  two  dissimilar  metals, 
instead  of  by  a  single  metal,  and  ascribed 
the  cause  of  the  electricity  produced  as 
the  contact  of  dissimilar  substances.  In 
the  light   of   more   modern   scientific  dis- 


ELECTRO-THERAPEUTICS.  7 

covery,  it  would  appear  that  both  dis- 
coverers saw  but  a  partial  truth,  although 
Volta  had  undoubtedly  a  more  complete 
grasp  of  the  phenomena  than  Galvani. 
Galvani  observed  the  convulsive  move- 
ments, but  improperly  attributed  them  to 
the  presence  of  a  vital  fluid.  Volta  cor- 
rectly ascribed  the  cause  of  the  movements 
to  the  passage  of  electricity,  but  incor- 
rectly ascribed  the  cause  of  the  continuous 
supply  of  electric  current  to  the  contact 
of  dissimilar  substances.  Modern  research 
has  shown  that  contact  alone  is  unable  to 
account  for  the  continuous  production  of 
an  electric  current,  and  that  such  a  dis- 
charge occurs  only  under  circumstances 
when  chemical  actions  take  place. 

In  endeavoring,  at  the  present  time,  to 
gauge  the  value  to  the  world  of  the  dis- 
coveries of  these  two  pioneer  investigators, 


8  ELECTRICITY   IN 

a  tendency  may  exist  to  give,  too  unre- 
servedly, the  award  to  Volta,  on  the  plea 
that  the  result  of  his  investigations  pro- 
duced, some  ten  years  later,  the  great  dis- 
covery of  the  Voltaic  pile,  a  discovery 
which  has  done  so  much  for  the  world's 
progress,  but  it  must  not  be  forgotten 
that  it  was  the  original  observations  of 
Galvani,  aided,  it  is  true,  largely  by  his 
afterwork  in  the  same  field,  that  first 
called  attention  to  the  wonderful  effects 
which  electricity  produces  on  the  animal 
organism,  and  if  to-day  electro-therapeu- 
tics, or  the  application  of  electricity  for 
the  restoration  of  the  healthy  condition 
of  the  body,  is  an  actual  power,  the 
beneficial  effects  of  which  are  apparently 
being  more  and  more  clearly  established 
every  year,  it  is  undoubtedly  to  Galvani  that 
the  guerdon  of  the  discovery  must  be 
awarded. 


ELECTRO-THERAPEUTICS.  9 

Without  attempting  to  trace  the  history 
of  the  extended  experiments  that  were 
made  on  Galvani's  original  observation, 
experiments  that  have  continued  up  to  the 
present  day,  and  without  stopping  to  con- 
sider the  extended  and  bitter  controversy 
that  was  waged  between  the  disciples  of 
Galvani  on  one  side,  and  those  of  Volta  on 
the  other,  as  to  the  cause  of  the  phenomena, 
or  the  equally  extended  controversy  which 
existed  as  to  the  origin  of  the  electric 
current  produced  in  the  voltaic  pile,  or  cell, 
it  will  suffice,  for  our  present  purpose,  to 
consider  animals  as  electric  sources,  and 
the  effects  produced  by  the  passage  of 
electricity  through  animals.  These  can  be 
briefly  summarized  as  follows ;  viz., 

(1)  That  the  body  of  an  animal  is,  in 
itself,  the  seat  of  electric  currents. 

(2)  That  these  currents  exist  not  only 
during  the  abnormal  or  diseased  condition 


10  ELECTRICITY  IN 

of  different  parts  of  the  body,  but  also  in 
the  normal  condition  of  the  body. 

(3)^  That  electric  discharges,  when  sent 
through  the  body  of  an  animal,  are  cap- 
able of  producing  marked  effects  therein, 
the  character  of  which  depends  upon 
the  nature  of  the  discharge. 

(4)  That  the  passage  of  an  electric  dis- 
charge through  a  nerve,  muscle,  or  indeed 
through  any  organ  of  the  body  of  an  ani- 
mal, produces  an  alteration  in  its  functional 
activity. 

(5)  That  a  sufficiently  powerful  dis- 
charge through  the  body  of  an  animal  may 
produce  death. 

The  effects  of  electricity  on  the  human 
body  have  been  very  generally  recognized 
since  the  time  of  Galvani,  and  it  is  gener- 
ally believed  that  electricity  possesses 
powerful   remedial   properties ;    but,   like 


all  remedra^  agencies,  unless  lnteilige 
applied,  it^^f^groduce  more  ha^rti^^m 
good.  That  ma^J^^MsrT^en^«h5ne  by 
its  improper  use  there  can  be  no  doubt. 
Indeed,  one  of  the  results  of  such  misuse 
has  been  even  at  the  present  time  to 
greatly  retard  its  general  introduction. 
Much  of  this  difficulty  has  arisen  from 
want  of  a  general  knowledge  of  the  funda- 
mental laws  underlying  the  production 
and  action  of  electricity  and  magnetism. 
As  a  result  of  a  lack  of  such  knowledge, 
extravagant  and  ridiculous  claims  are  fre- 
quently made  as  to  the  wonderful  curative 
powers  of  certain  apparatus,  alleged  to 
produce  electric  currents ;  apparatus  that 
even  a  tyro  in  electricity  would  at  once  be 
able  to  show  are  incapable  of  producing 
any  current  whatever. 

A  knowledge  of  the  laws  of  electricity 


12  ELECTRICITY. 

and  magnetism  will,  therefore,  not  only 
enable  the  general  public  to  detect  elec- 
tro-therapeutic frauds,  but  will  also  tend 
to  increase  confidence  in  legitimate  elec- 
tro-therapeutic applications,  a  confidence 
justly  merited  by  experience. 

Since  both  electricity  and  magnetism 
are  exact  sciences,  their  application  to  the 
art  of  electro-therapeutics  cannot  fail  to  be 
of  benefit  in  scientific  treatment. 


CHAPTER  II. 

ELECTROMOTIVE    FOECE. 

Although  we  are  ignorant  of  the  exact 
nature  of  electricity,  yet  it  is  by  no  means 
true  that  we  are  ignorant  of  the  laws 
under  which  electricity  operates.  In 
other  words,  our  ignorance  relates  to  the 
exact  nature  of  the  electric  force  rather 
than  to  its  laws.  In  the  physical  world  we 
can  properly  claim  knowledge  concerning  a 
force,  when  we  are  able  to  predict  its 
action  under  given  conditions.  Gauged  in 
this  way,  our  knowledge  of  electricity  is 
both  extensive  and  accurate,  since  it  is 
possible  to  fairly  predict  just  what  will 
happen  under  a  great  variety  of  electric 
conditions. 

13 


14  ELECTRICITY  IN 

It  will  be  generally  acknowledged,  how- 
ever, that  we  certainly  know  this  much 
about  electricity ;  viz.,  that  it  cannot  be 
regarded  as  a  form  of  matter ;  at  least 
not  matter  in  the  ordinary  sense  of  the 
word.  All  matter  with  which  we  are 
acquainted,  exercises  gravitational  influ- 
ence, that  is  to  say,  tends  to  attract  other 
matter  towards  it.  No  such  tendency  has 
yet  been  shown  to  exist  in  the  case  of 
electricity.  But  ordinary  matter  is  not 
the  only  material  with  which  we  are  sur- 
rounded. The  entire  universe  is  believed 
to  be  pervaded  by  a  highly  tenuous 
medium  called  the  ether,  which  transmits 
light,  heat  and  gravitation. 

As  frequent  reference  will  be  made  in 
this  book  to  the  existence  and  properties 
of  the  ether,  it  may  be  well  to  explain  the 
nature    of  the    evidence   which    has   con- 


ELECTRO-THERAPEUTICS.  15 

vinced  scientific  men  of  its  existence.  We 
know  that  the  effects  produced  by  a 
sounding  body,  such,  for  example,  as  a 
vibrating  bell,  are  transmitted  across  the 
space  between  the  bell  and  the  observer's 
ear,   by   means    of    waves,   or    to-and-fro 


Fig.  2.— Transmission  of  Sound  through  a  Vacuum. 

motions  produced  in  the  medium  existing 
between  the  bell  and  the  ear.  This 
medium  is  usually  the  air.  If  a  bell, 
placed  inside  a  glass  vessel,  as  shown  in 
Fig.  2,  be  set  vibrating,  it  can  be  heard  by 
an  observer  at  some  distance,  since  the  air 
the  vessel  contains  transmits  the  vibrations 
to  the  sides  of  the  globe,  which,  in  their 


16  ELECTRICITY  IN 

turn,  transmit  the  vibrations  to  the  external 
air,  and  so  to  the  observer's  ear.  But  if  the 
vessel  be  exhausted ;  i.  e.,  deprived  of  its 
air,  the  sound  will  no  longer  be  trans- 
mitted, since  the  medium  is  then  removed 
which  carries  its  vibrations.  If,  however, 
a  similar  experiment  be  tried  with  a  hot 
body,  it  will  be  found  that  the  existence 
of  such  a  "medium  as  air  is  not  essential  to 
permit  the  body  to  transmit  its  heat  across 
intervening  space.  For  example,  if,  as  in 
Fig.  3,  two  reflectors  A  and  B,  be  placed 
inside  the  glass  receiver  of  an  air-pump, 
and  a  delicate  thermometer  Ty  be  suitably 
supported  at  the  focus  of  one  of  these 
reflectors,  and  a  platinum  wire  be  placed 
at  the  focus  of  the  other  reflector,  then, 
if  an  electric  current  be  sent  through  the 
platinum  wire,  of  such  strength  as  to  ren- 
der it  incandescent,  the  heat  radiated  from 
the  wire  will  be  reflected  successively  from 


ELECTRO-THERAPEUTICS. 


17 


A  and  J?,  and  be  focused  on  the  ther- 
mometer, which  will  immediately  indicate 
an  increased  temperature,  and  this  effect 
will  occur,  whether  the  receiver  contains 


■**#* 


%3B^ 


p* 


Fig.  3.    Transmission  of  Heat  through  a  Vacuum. 


air  or  is  exhausted.  Evidently,  the  pres- 
ence of  a  gross  medium  like  air  is  not 
essential  to  the  transmission  of  radiant 
heat,  and  in  this  respect  differs  from  the 
transmission  of  sound  just  referred  to. 
Moreover,  the  light  emitted  by  the  glow- 


18  Electricity  in 

ing  platinum  wire  is  also  transmitted 
through  the  empty  space  in  the  globe  and 
renders  the  wire  visible. 

A  little  reflection  will  show  that  the 
preceding  experiment  is  not,  in  reality, 
needed  to  prove  the  possibility  of  radiant 
light  and  heat  being  readily  transmitted 
across  space  devoid  of  ordinary  matter ;  for, 
radiant  light  and  heat  reach  the  earth  from 
the  sun,  and  from  the  fixed  stars,  across 
space  existing  between  the  earth  and  the 
heavenly  bodies,  which  space  we  believe  to 
be  devoid  of  ordinary  matter.  In  the  early 
history  of  physical  science  this  fact  led  to 
the  belief  that  light  and  heat  were  effects 
produced  by  specific  fluids  or  effluvia  ; 
that  a  hot  body  sent  off  a  specific  efflu- 
vium which  constituted  heat ;  and,  in  a 
similar  manner,  a  luminous  body  sent  off  a 
specific  effluvium  constituting  light.     The 


ELECTRO-THERAPEUTICS.  19 

particular  effluvium  in  the  case  of  heat 
received  the  name  of  caloric,  a  term 
which,  unfortunately,  even  to-day,  is  still 
loosely  employed  in  the  science  of  heat. 
Without  entering  into  minute  details,  it 
suffices  to  say  that  the  theory  which 
ascribes  light  or  heat  to  the  existence  of 
special  fluids  is  now  considered  absolutely 
untenable.  Light  and  heat,  like  sound, 
are  believed  to  be  produced  by  vibrations, 
or  to-and-fro  motions.  A  medium,  there- 
fore, is  necessary  to  carry  the  vibrations 
of  light  and  heat,  and,  consequently,  the 
highest  vacuum  which  can  be  obtained  is, 
for  this  reason,  believed  to  be  filled  with 
ether,  that  is  to  say,  the  ether  is  not 
pumped  out  of  a  reservoir  by  the  action 
of  the  air-pump. 

The  ether  is  sometimes  called  the  lumi- 
niferous    ether    because    it   transmits  the 


20  ELECTRICITY   IN 

vibrations  of  light.  It  is  also  called  the 
universal  ether  because  it  is  supposed  to 
exist  everywhere.  Even  in  the  densest 
of  solid  bodies  it  is  assumed  to  exist, 
between  the  ultimate  particles,  of  which 
such  bodies  are  formed ;  namely,  between 
the  atoms  and  the  molecules. 

Electricity  and  magnetism,  like  heat  and 
light,  are  also  capable  of  manifesting  their 
influence  through  the  air-pump  vacuum. 
For  example,  the  filament  of  an  incandes- 
cent lamp,  which,  as  is  well  known,  is 
placed  in  a  very  high  vacuum,  can,  never- 
theless, be  deflected  by  the  electric 
attraction  produced  by  a  charged  body 
A,  Fig.  4,  at  some  distance  from  the 
lamp.  Here  the  electric  attraction  tra- 
verses the  apparently  empty  space  sur- 
rounding the  filament,  and,  consequently, 
must  have  acted  through  the  ether  which 


Fig.  4. — Transmission  of  Electrostatic  Force 
Across  a  Vacuum. 

is    believed    to  fill    the    exhausted   lamp 
chamber. 


In  a  similar  manner,  magnetic  attraction 
is  capable  of  acting  through  empty  space  ; 


22 


ELECTRICITY   IN 


for,  if,  as  in  Fig.  5,  an  incandescent  electric 
lamp  be  brought  near  a  powerful  magnet, 
when    a    continuous    current     is    passing 


Warn    ^HI^^^^^^^^Hh 


Fig.  5. — Transmission  of  Magnetic  Force  Across 
a  Vacuum. 

through  the  filament,  the  filament  has 
thereby  acquired  magnetic  properties,  and 
a  deflection  of  the  filament  will  take  place, 


ELECTRO-THERAPEUTICS.  23 

although  no  medium,  save  the  ether,  can 
apparently  convey  this  influence  within 
the  chamber.  Here,  as  before,  this  par- 
ticular experiment  is  not  necessary  to 
show  that  magnetic  influence  can  traverse 
apparently  empty  space  ;  for,  during  the 
prevalence  of  an  unusual  number  of  spots 
on  the  surface  of  the  sun,  there  are  pro- 
duced marked  disturbances  on  delicately 
suspended  compass  needles  on  the  earth. 
This  influence  is  apparently  transmitted 
through  the  ether  which  we  believe  Alls 
interstellar  space. 

It  is  interesting  to  note  in  this  connec- 
tion, that  the  early  views  concerning  the 
nature  of  electricity  regarded  it  as  a  fluid 
or  fluids,  just  as  heat  and  light  were 
originally  regarded.  It  is  now  the  belief, 
however,  that  electricity  and  magnetism 
are  phenomena  connected  with  some  active 


24  ELECTRICITY   IN 

condition  of  the  ether.  For  example,  light 
is  almost  universally  regarded  as  being 
transmitted  by  a  particular  transverse  vi- 
bration of  the  ether,  and  some  particular 
forms  of  disturbances  in  the  ether  are  also 
believed  to  be  the  causes  of  electric  and 
magnetic  phenomena. 

Before,  however,  discussing  at  greater 
length  the  nature  of  electricity,  let  us  con- 
sider a  well-known  electric  source,  as, 
for  example,  the  dynamo-electric  machine, 
such  as  is  used  for  generating  electric  cur- 
rents for  arc  or  incandescent  lights.  Here, 
popularly,  the  machine  is  spoken  of  as  pro- 
ducing electricity.  Strictly  speaking,  how- 
ever, it  is  not  electricity  which  the  machine 
primarily  produces.  The  machine  produces 
a  variety  of  force  capable  of  starting,  or 
producing,  an  electric  current  under  suit- 
able conditions.     This  force  produced  by 


ELECTRO-THERAPEUTICS.  25 

the  dynamo  is  termed  electromotive  force, 
or  the  force  which  tends  to  set  electricity 
in  motion,  so  as  to  cause  an  electric  now. 
It  is  essential  to  bear  in  mind  that  in  no 
case  can  electricity  be  produced  by  any 
machine  without  the  prior  production  of 
electromotive  force,  just  as  no  motion  can 
exist  in  any  material  object  without  the 
antecedent  application  of  a  material  force ; 
i.  e.y  of  a  body-moving  force.  Whenever, 
therefore,  electricity  is  produced,  no  matter 
what  the  nature  of  the  machine,  or  source 
producing  it  may  be,  the  machine  or 
source  must  necessarily  first  produce  an 
electromotive  force,  and  this  electromotive 
force  in  its  turn  will  or  will  not  produce 
electricity,  according  to  the  conditions 
under  which  it  acts.  Electromotive  force 
is  usually  abbreviated  E.  M.  F. 

Various  devices  are  employed  in  prac- 


ELECTRICITY   IN 


tice  for  the  production  of  E.  ML  F.     Such 
devices  may  be  classified  as  follows : 

(1)  Those  produced  by  chemical  action ; 
such  as  a  voltaic  cell  or  primary  cell,  and 
a  charged  storage  cell  or  secondary  cell. 

(2)  Those  produced  by  the  action  of 
radiant  energy ;  i.  e.,  radiant  light  or  heat ; 
such,  for  example,  as  a  thermo-electric  cell. 

(3)  Those  produced  by  the  action  of 
mechanical  energy ;  such,  for  example,  as 
a  dynamo-electric  machine,  a  frictional 
machine,  an  electrostatic  induction  machine, 
or  a  liquid  flowing  through  a  capillary 
tube. 

(4)  Those  produced  by  vital  energy, 
such  as  an  animal  or  a  plant  regarded  as  an 
electric  source. 

As  already  stated,  in  any  of  the  preced- 
ing sources  it  is  E.  M.  F.  which  is  pri- 
marily produced. 


ELECTRO-THERAPEUTICS.  27 

Take,  for  example,  a  form  of  voltaic  cell, 
shown  in  Fig.  6,  known  as  the  Leclanche 


Fig.  6.— Leclanche  Voltaic  Cell. 

cell.  Here  it  is  the  chemical  energy  of 
combination  between  the  zinc  plate  A,  and 
the  solution  of  ammonium  chloride,  which 
enables  the  electric  current  to  be  sustained, 
but  the  cell  always  produces  an  E.  M.  F., 


28  ELECTRICITY  IN 

although  it  will  not  supply  an  electric  cur- 
rent until  its  terminals  A  and  B,  are  con- 
nected together  by  means  of  a  conductor 
or  external  circuit. 

In  order  to  measure  the  E.  M.  F.  of 
any  electric  source,  a  unit  of  E.  M.  F. 
has  been  internationally  adopted.  This 
unit  is  called  the  volt,  after  Alessandro 
Volta,  the  inventor  of  the  voltaic  cell. 
The  E.  M.  F.  of  the  Leclanche  cell  shown 
in  Fig.  6,  is,  approximately,  1\  volts, 
and  that  of  the  ordinary  blue-stone  cell, 
as  shown  in  Fig.  7,  is  about  one  volt. 

In  both  the  Leclanche  and  the  blue- 
stone  voltaic  cells,  it  will  be  observed  that 
there  are  two  metallic  substances  immersed 
in  a  liquid.  For  example,  in  the  Le- 
clanche cell,  shown  in  Fig.  6,  the  two  sub- 
stances are  carbon  and  zinc,  and  the  solu- 


ELECTRO-THERAPEUTICS.  29 

tion  in  which  they  are  plunged  is  an  aque- 
ous solution  of  sal-ammoniac.  In  the  blue- 
stone,  or  gravity  cell,  shown  in  Fig.  7,  the 


Fig.  7.— Bluestone  or  Gravity  Voltaic  Cell. 

two  metals  are  zinc  and  copper,  marked 
Zn  and  On,  but  here  there  are  two  sepa- 
rate exciting  liquids  ;  namely,  a  dense  solu- 
tion of  copper  sulphate,  which  occupies  the 
lower  part  of  the  cell,  and  a  lighter  solu- 
tion of  zinc  sulphate,  which  surrounds  the 


30  ELECTRICITY   IN 

zinc  plate  and  floats  upon  the  copper  sul- 
phate solution.  All  voltaic  cells  may  be 
divided  into  two  general  classes ;  namely, 

(1)  The  single-fluid  cells,  or  those  which 
have  a  single  exciting  fluid ;  and, 

(2)  The  double-fluid  cells,  or  those 
which,  like  the  bluestone  cell,  have  two 
exciting  fluids. 

Every  voltaic  cell,  whether  of  the 
double-  or  single-fluid  type,  consists  of  two 
essential  parts ;  namely, 

(1)  Of  a  voltaic  pair  or  voltaic  couple, 
consisting  of  two  dissimilar  electrically  con- 
ducting substances. 

(2)  Of  an  exciting  liquid  called  the  elec- 
trolyte, capable  of  conducting  electricity, 
and  of  being  decomposed  by  it.  The 
double-fluid  cells  have  two  liquids  or 
electrolytes.  The  two  substances  forming 
a  voltaic  pair    or   couple,  are  called    the 


ELECTRO-THERAPEUTICS.  31 

elements  of  the  cell.  Voltaic  elements  are 
generally  made  in  the  form  of  plates  or 
rods,  and  are  known  respectively  as  the  posi- 
tive and  the  negative  plates  or  elements. 

During  the  action  of  a  voltaic  cell,  that 
is,  while  it  is  furnishing  electric  current 
to  the  circuit  connected  with  it,  a  chemical 
action  takes  place  between  one  or  both  of 
the  electrolytes  and  one  of  the  plates. 
The  result  of  this  action  is  that  one  of  the 
plates,  the  positive,  is  gradually  dissolved, 
or  enters  into  chemical  combination  with 
part  of  the  electrolyte,  the  other  plate 
remaining  unacted  on.  In  nearly  all  forms 
of  voltaic  cells,  there  results  from  this 
decomposition  a  tendency  to  liberate  hydro- 
gen at  the  surface  of  the  negative  plate,  or 
the  plate  which  is  unacted  on.  If  hydro- 
gen be  permitted  to  be  liberated  on  the 
surface  of  the   negative   plate,  a  marked 


32  ELECTRICITY  IN 

decrease  occurs  in  the  ability  of  the  cell 
to  furnish  current,  for  reasons  which  will 
be  pointed  out  hereafter. 

In  single-fluid  cells  no  provision  is  made 
to  prevent  the  evolution  of  hydrogen  at 
the  negative  plate ;  or,  as  it  is  generally 
called,  the  polarization  of  the  negative 
plate.  In  the  double-fluid  cell  the  second 
fluid  consists  of  a  substance  which  sur- 
rounds the  negative  plate,  and  is  provided 
for  the  express  purpose  of  entering  into 
combination  with  the  hydrogen  and  so 
preventing  its  being  liberated.  There  are 
some  forms  of  voltaic  cells  which  are  ap- 
parently single-fluid  cells,  since  they  possess 
but  a  single  fluid,  or  electrolyte,  but  which 
properly  come  under  the  type  of  double- 
fluid  cells,  since  they  are  uon-polarizable, 
being  provided  with  a  solid  substance  in 
contact  with  the   negative  plate,  that   is 


ELECTRO-THERAPEUTICS.  33 

capable  of  combining  with  hydrogen  and 
thereby  preventing  its  liberation.  For 
example,  in  the  Leclanche  cell,  shown  in 
Fig.  6,  the  elements  of  the  voltaic  couple  are 
zinc  and  carbon,  immersed  in  a  solution  of 
sal-ammoniac  in  water,  but  the  carbon  is 
surrounded  by  granulated,  solid  peroxide 
of  manganese,  which  possesses  the  power 
of  readily  entering  into  combination  with 
hydrogen. 

A  great  variety  of  conducting  sub- 
stances are  employed  in  pairs  for  the 
couples  of  voltaic  cells.  The  most  impor- 
tant of  these,  however,  are  zinc,  carbon, 
copper,  lead,  silver,  and  platinum.  Of 
these  substances,  zinc  in  nearly  all  cases 
forms  the  positive  element ;  that  is  to 
say,  it  forms  a  voltaic  couple  either 
with  carbon,  copper,  lead,  silver,  or 
platinum. 


34  ELECTRICITY   IN 

When  a  voltaic  cell  Las  its  circuit  closed, 
for  example,  when  the  zinc-carbon  couple 
shown  in  Fig.  6,  is  connected  to  an  exter- 
nal circuit,  the  E.  M.  F.  it  produces  causes 
a  current  to  flow  through  such  circuit. 
For  purposes  of  convenience  it  has  been 
agreed,  conventionally,  to  regard  the  electric 
current  as  leaving  a  voltaic  cell  at  a  par- 
ticular point,  and,  after  passing  through 
the  circuit,  to  re-enter  it  at  another  point. 
The  point  at  which  the  current  is  conven- 
tionally assumed  to  leave  the  cell  is  called 
its  positive  pole,  and  the  point  at  which 
it  is  assumed  to  re-enter  it,  after  having 
passed  through  the  circuit,  the  negative 
pole.  The  positive  pole  of  the  cell  is  the 
pole  connected  with  the  plate  which  is  not 
acted  on,  that  is  with  the  negative  plate, 
while  the  negative  pole  is  the  pole  con- 
nected with  the  plate  which  is  chemically 
acted    on,   or    the    positive    plate.       For 


ELECTRO-THERAPEUTICS.  35 

example,  in  the  battery  shown  in  Fig.  7, 
the  positive  pole  is  connected  with  the 
copper  plate,  and  is  marked  with  a  plus, 
and  an  arrow,  indicating  the  fact  that  the 
current  leaves  the  cell  at  this  pole ;  while 
the  negative  pole  is  the  terminal  of  the 
zinc  plate,  and  is  indicated  by  a  minus 
sign,  and  an  arrow  flowing  towards  the 
cell,  indicating  the  fact  that  the  current 
enters  the  cell  at  this  pole.  In  the  battery 
shown  in  Fig.  6,  the  positive  pole  is  the 
terminal  B,  of  the  carbon  element,  while 
the  negative  pole  is  the  terminal  A,  of  the 
zinc  element. 

Voltaic  cells  form  an  important  electric 
source  much  employed  in  electro-therapeu- 
tics. We  will,  therefore,  briefly  describe 
some  of  their  more  important  practical 
forms. 

Fig.   8,  shows  a  form    of    voltaic   cell, 


36 


ELECTRICITY   IX 


called   the  silver-chloride   cell.     This  cell 
consists  of  a  zinc-silver  couple,  immersed 


Fig.  8. — Form  of  Silver-Chloride  Cell. 

in  a  dilute  aqueous  solution  of  sal-am- 
moniac. The  silver  plate  has  the  form  of 
a  wire,  and  is  surrounded  by  a  fused  mass 
of  silver   chloride.     The   arrangement   of 


ELECTRO-T 


Fig.  9.— Silver-Chloride  Voltaic  Cell. 


38  ELECTRICITY   IN 

the  plates  is  shown  in  the  figure.  It  will 
be  seen  that  a  thread  B,  is  wrapped  around 
the  silver  and  silver  chloride,  so  as  to  pre- 
vent the  possibility  of  contact  between  the 
silver  element  and  the  zinc  plate.  The 
two  plates  are  also  kept  apart  by  a  small 
block  of  wood  W.  The  couple  so  formed 
is  placed  in  a  small  glass  or  rubber  jar  J, 
containing  the  exciting  solution  of  sal- 
ammoniac.  Another  form  of  silver- 
chloride  cell  is  shown  in  Fig.  9.  The 
advantage  of  the  silver-chloride  cell  con- 
sists in  its  portability.  As  many  as  fifty 
of  these  cells  can  be  set  in  a  frame,  as 
shown  in  Fig.  10,  and  enclosed  in  a  small 
wooden  box  weighing  only  ten  pounds. 
The  silver-chloride  cell  is  very  nearly  uni- 
form in  its  electromotive  force,  which  has  a 
value  of  about  1.03  volts.  The  cell  pos- 
sesses, however,  the  disadvantage  of  not 
being    able  to    supply    powerful  currents 


ELECTRO-THERAPEUTICS.  39 

continuously,  owing  to  the  fact  that  in 
order  to  obtain  portability,  its  elements  are 
made  so  small.  Were  the  cell  constructed 
of  such  a  size  as  would  permit  it  to  supply 


Fig.  10.— Battery  of  Silver-Chloride  Cells. 

powerful  currents,  the  cost  of  the  silver 
and  silver-chloride  would  render  its  use 
impracticable.  The  cell  is  particularly 
well  adapted  to  supply  feeble  currents, 
requiring  a  considerable  E.  M.  F.,  espe- 
cially when  portability  is  desired. 


40 


ELECTRICITY   IN 


Fig.  11,  shows  a  single-fluid  cell,  consist- 
ing of  a  zinc-carbon  couple  in  an  exciting 


Fig.  11. — Zinc- Carbon  Cell. 

solution  of  sal-ammoniac  in  water.  Here, 
as  before,  the  terminal  of  the  zinc  plate 
forms  the  negative  pole  and  that  of  the 


ELECTRO-THERAPEUTICS.  41 

carbon  plate,  formed  of  a  number  of 
rods  of  carbon,  the  positive  pole.  This 
cell  furnishes  a  strong  current  for  a  short 
time.  Its  advantage  consists  in  the  fact 
that  it  will  supply  a  moderately  strong 
current  for  a  brief  interval,  and  that  it 
suffers  very  little  chemical  loss  on  open 
circuit.  Its  E.  M.  F.  is  about  1J£  volts. 
The  disadvantage  of  the  cell  is  that  it 
polarizes  considerably,  so  that  it  cannot 
continue  to  furnish  current  of  any  con- 
siderable strength  for  a  long  time. 

Fig.  12,  shows  another  form  of  a  couple 
of  zinc-carbon  immersed  in  a  solution 
called  electropoion  fluid,  consisting  of 
chromic  acid  and  water,  or  of  bichromate 
of  potash,  sulphuric  acid  and  water.  This 
cell  is  called  the  Grenet  or  bichromate  cell. 
The  advantage  of  the  Grenet  cell  is  that  it 
is  capable  of  supplying  a  fairly  strong  cur- 


42  ELECTRICITY   IN 

rent  for  a  short  time,  though  longer  than 
in  the  case  of  the  simple  carbon-zinc  cell. 
Its  disadvantage  is  that  the  chemical  action 


Fig.  12.— Grenet  Plunge  Cell. 

continues  even  when  the  cell  is  on  open 
circuit,  so  that  the  zinc  becomes  dissolved. 
Consequently,  in  practice  provision  has  to 
be  made  in  ■  this  cell,  for  raising  the  zinc 


ELECTRO-THERAPEUTICS.  43 

plate  from  the  solution  when  the  battery  is 
not  in  use. 


Fig.  13,  shows  two  different   forms  of 


Fig.  13.— Edison-Lalande  Cells. 

Edison-Lalande  cell.  Here  the  couple  is 
formed  of  plates  of  zinc  and  copper  im- 
mersed in  a  solution  of  caustic  soda,  or 
potash,  in  water.     Although  this  cell  em- 


44  ELECTRICITY   IN 

ploys  but  a  single  liquid,  yet,  like  the 
Leclanche  cell,  a  special  provision  is  made 
to  prevent  polarization.  This  is  accom- 
plished by  placing  a  plate  of  compressed 
copper  oxide  in  contact  with  the  copper 
plate,  so  that  the  hydrogen,  which  tends 
to  be  liberated  at  the  surface  of  the 
copper  plate,  is  prevented  from  doing  so 
by  entering  into  combination  with  the 
oxygen  of  the  oxide  of  copper.  The  Edi- 
son-Lalande  cell  possesses  the  advantage  of 
furnishing  powerful  currents  for  a  con- 
siderable length  of  time  without  sensible 
polarization,  and  also  of  suffering  negligi- 
ble local  action  or  chemical  loss  on  open 
circuit.  Its  disadvantage  lies  in  its  low  E. 
M.  F.,  which  is  only  about  two  thirds  of  a 
volt,  when  at  work. 

Another  form  of  Edison-Lalande  cell  is 
shown  in  Fig.  14.     Here    a  large  copper 


ELECTRO-THERAPEUTICS.  45 


Fig.  14.    Edison-Lalande  Cautery  Cell. 

plate   is  placed  between  two   zinc  plates. 
The    object  of   this    form   of    cell    is    to 


46 


ELECTRICITY   IN 


provide    powerful    currents    suitable    for 
heating  electric    cauteries;    i.   e.,   metallic 


Fig.  15.— Pahtz  Gravity  Cell. 

wires  or  strips  raised  to  a  white  heat  by 
the  passage  of  an  electric  current,  and 
employed  for  removing  diseased  growths. 


ELECTRO-THERAPEUTICS.  47 

Fig.  15,  shows  a  form  of  zinc-carbon 
cell,  called  the  Partz  gravity  cell.  This  is 
a  double-fluid,  gravity  cell,  the  fluids  be- 
ing a  solution  of  common  salt,  or  sulphate 
of  magnesia,  and  a  dense  solution  of  a  salt 
called  sulpho-chromic  salt,  practically  a 
salt  which,  dissolved  in  water,  forms  the 
electropoion  solution  before  referred  to. 
In  order  to  charge  the  cell,  the  jar  is  first 
partly  filled  either  with  a  solution  of  com- 
mon salt  or  magnesium  sulphate,  and  the 
sulpho-chromic  salt  added  through  the  fun- 
nel shaped  tube  on  the  left-hand  side  of 
the  cell,  thus  forming  a  dense  solution  sur- 
rounding the  lower  or  carbon  plate.  The 
advantage  of  this  cell  is  its  high  E.  M. 
F.,  nearly  two  volts,  and  the  strength  of 
current  it  can  supply.  Its  disadvantage 
is  local  action  on  open  circuit. 

Fig.  16,  shows  a  form  of  cell   called  a 


48  ELECTRICITY   IN 

dry  cell.  This  name  is  badly  chosen, 
since,  although  the  cell  does  not  actually 
contain  free  liquid,  yet  its  action  is  depend- 


Fig.  16.— Form  of  Dry  Cell. 

ent  upon  the  presence  of  a  liquid  electro- 
lyte, in  this  case  the  liquid  being  absorbed 
either  by  a  gelatinous  substance,  or  by 
some  pulverulent  material.  The  advan- 
tage   of   a   cell    which    contains    no   free 


ELECTRO-THERAPEUTICS.  49 

liquid,  is  that  the  cell  can  be  readily  car- 
ried about  without  spilling  any  of  the  ex- 
citing liquid.  The  disadvantage  of  the  cell 
is  that  the  current  it  will  supply  is  compar- 
atively feeble,  owing  to  its  high  resistance. 

Dry  cells  give  E.  M.  Fs.  varying  from 
two-third  volt,  to  about  two  volts. 

Where  the  circuit  to  be  supplied  is 
such  that  the  E.  M.  F.  furnished  by  a 
single  cell  is  not  capable  of  overcoming 
what  is  called  the  resistance  of  the  circuit, 
it  is  necessary  to  connect  a  number  of 
separate  cells  so  that  they  may  all  supply 
their  currents  into  the  same  circuit.  A 
number  of  separate  cells  capable  of  acting 
as  a  single  cell  is  called  a  battery,  a  term  fre- 
quently incorrectly  applied  to  a  single  cell. 

A  number  of  voltaic  cells  may  be  con- 
nected to  form  a  battery,  in  a  variety  of 


50  ELECTRICITY   IK 

ways,  but  at  present  we  will  discuss  only 
one  of  such  connections ;  namely,  connec- 
tion in  series.  The  method  adopted  in 
this  connection  is  shown  in  Fig.  17,  where 
three  Daniell  gravity  cells  are  shown,  con- 


Fig.  17.— Series  Connection  of  Three  Daniell 
Gravity  Cells. 

nected  in  series.  Here  the  negative  pole 
of  the  cell  A,  is  connected  with  the  posi- 
tive pole  of  the  cell  B ;  the  negative  pole 
of  By  is  connected  with  the  positive  pole 
of  O,  while  the  free  positive  and  the  free 
negative  poles  of  A  and  C\  respectively, 
form  the  terminals  of  the  battery.     In  the 


ELECTRO 


case  of  the  series 
cells,  the  E.  M.  F. 
the  sum  of  the  E.  M. 


Fig.  18.— Edison-Lalande  Battery. 

cells  composing  it.  Consequently,  the 
battery  shown  in  Fig.  17,  will  have  an  E.  M. 
F.  of  approximately  three  times  1.05  volts. 

Fig.  18,  shows  a  battery  of  two  series- 
connected,  Edison-Lalande  cells.     Fig.  19, 


52 


ELECTRICITY   IN 


shows  a  battery  of  the  general  type  shown 
in  Fig.  12.  Here  each  cell  is  formed  of  a 
number  of  plates  of  zinc  and  carbon. 
When  the  battery  is  desired  for  use,  the 


rrrrrrrrrfrf 


HUIU 


Fig.  19.— Plunge  Battery. 


handle  is  turned  so  that  the  plates  are  let 
down  into  the  jars  filled  with  the  exciting 
liquid.  When  out  of  use,  the  plates  are 
raised  from  the  jars,  in  order  to  preserve 
them    from    corrosion,    or     local    action. 


ELECTRO-THERAPEUTICS.  53 

Should  the  plates  not  be  removed  from 
the  exciting  liquid,  the  zincs  will  rapidly 
be  corroded. 

Fig.  20,  shows  a  battery  of  50  series- 
connected,  silver-chloride  cells,  so  arranged 


Fig.  20.— Silver-Chloride  Battery  of  Fifty  Cells. 


54  ELECTRICITY  IN 

that  any  number  from  one  up  to  50  can  be 
connected  to  the  main  terminals. 

A  voltaic  cell  will  continue  to  furnish 
current  to  the  circuit  connected  with  it,  as 
long  as  it  contains  any  positive  metal  to  be 
dissolved,  and  any  electrolyte  to  dissolve 
it.  As  soon  as  either  the  electrolyte  or  the 
positive  plate  is  consumed,  the  cell  ceases 
to  give  current  until  it  is  furnished  with 
another  plate  or  with  more  electrolyte,  or 
both.  In  the  form  of  voltaic  cell  called 
the  storage  cell,  or  secondary  cell,  to  dis- 
tinguish it  from  the  ordinary  voltaic  or 
primary  cell,  the  selection  of  the  elements 
of  the  couple  and  the  electrolyte  are  such, 
that  after  the  cell  is  completely  exhausted 
or  run  down,  it  is  capable  of  being  restored 
or  charged,  and  of  again  being  brought 
into  a  condition  ready  for  action  by  the 
passage  of  an  electric  current  through  it  in 


ELECTRO-THERAPEUTICS.  55 

the  opposite  direction  to  that  of  the  cur- 
rent which  it  furnishes  when  charged. 
The  passage  of  this  charging  current  has 
the  effect  of  producing  a  series  of  decom- 
positions, which  practically  restore  the 
condition  both  of  the  elements  of  the 
couple  and  of  the  exciting  liquid  or  elec- 
trolyte. 

Secondary  or  storage  cells  are  made  in 
a  variety  of  forms,  but  those  in  common 
use,  consist  practically,  when  charged,  of  a 
voltaic  couple  of  porous  lead  and  lead 
peroxide,  immersed  in  an  electrolyte  of 
dilute  sulphuric  acid.  The  lead  peroxide 
forms  the  positive  plate,  and  the  metallic 
finely  divided,  porous  lead,  the  negative 
plate. 

In  most  forms  of  storage  battery,  the 
substances  forming  the  positive  and  nega- 


56 


ELECTRICITY   IN 


tive  elements  are  packed  in  perforations  in 
a  supporting  plate  or  grid,  made  of  an 
alloy  of  lead  containing  a  small  percentage 


Fig.  21. — Plate  of  Chloride  Storage  Battery. 

of  antimony.  The  object  of  the  antimony 
is  to  prevent  the  action  of  the  sulphuric 
acid  on  the  lead  in  the  grid  during  charg- 
ing. Fig.  21,  shows  a  plate  of  a  well- 
known   form   of   storage   cell,   called    the 


ELECTRO- 


chloride  storage  ce 
as  shown,  of  a  lead-a 


Fig.  22. — Interior  View  of  Chloride  Storage 
Battery. 

ing  pastels  or  discs  of  metallic  lead  or  lead 
peroxide,  according  to  the  nature  of  the 
plate.     A    number    of     such    plates    are 


58 


ELECTRICITY   IN 


generally  grouped  together,  to  form  a 
single  storage  cell,  the  connections  being 
such  that  all  the  positive  plates  are  con- 


Fig.  23.— Portable  Chloride  Storage  Battery. 


nected  together  to  form  a  single  positive 
plate,  and  all  the  negative  plates  are 
similarly  connected  to  form-  a  single  nega- 


ELECTRO-THERAPEUTICS.  59 

tive  plate,  the  whole  being  immersed  in  a 
jar  containing  a  solution  of  sulphuric  acid 
in  water.  In  Fig.  22,  two  of  such  storage 
cells  are  connected  together  to  form  a 
storage  battery.  A  portable  form  of 
chloride  storage  battery  suitable  for  medi- 
cal purposes  is  shown  in  Fig.  23. 

Another  form  of  storage  cell,  called  the 
Julien  Cell,  is  shown  in  Fig.  24.  The 
advantage  of  storage  cells  is  that  they 
are  capable  of  supplying  a  very  powerful 
current,  and  that  the  E.  M.  F.  of  each  cell 
is  two  volts.  Their  disadvantage  is  that 
they  require  to  be  periodically  charged. 

Storage  cells  are  generally  charged  by 
connecting  them  with  the  terminals  of  a 
dynamo-electric  machine,  the  positive 
terminal  being  connected  to  the  positive 
terminal   of    the    dynamo.     The    dynamo 


60  ELECTRICITY  IN 

may  be  provided  especially  for   the  pur- 
pose,   or    the    charging    current    may  be 


Fig.  24— Julien  Storage  Cell. 

obtained  from   the  mains  supplying  elec- 
tric incandescent  lamps.     Sometimes,  how- 


ELECTRO-THERAPEUTICS.  61 


Fig.  25. — Cabinet  Containing  Eleven  Primary  Vol- 
taic Cells  for  Charge  of  Two  Storage  Cells. 


62  ELECTRICITY. 

ever,  a  primary  battery  is  employed  for 
this  purpose.  For  example,  as  shown  in 
Fig.  25,  11  gravity  cells  are  connected  in 
a  battery  so  as  to  charge  two  storage  cells. 
The  current  supplied  by  the  primary  bat- 
tery is  a  feeble  but  steady  current,  and  the 
storage  cells  when  charged  by  the  action  of 
this  current,  may  be  many  times  stronger 
for  a  correspondingly  shorter  length  of 
time. 


CHAPTER  III. 

ELECTEIC    KESISTANCE. 

We  have  pointed  out,  in  the  preceding 
chapter,  that  it  is  not  electricity  which  an 
electric  source  primarily  produces,  but  an 
electromotive  force,  or  a  force  capable  of 
producing  a  flow  or  current  of  electricity, 
when  the  terminals  or  poles  of  the  source 
are  suitably  connected  by  a  conducting 
path  or  circuit.  We  have  also  seen  that 
electromotive  force  is  measured  in  units 
called  volts,  and  that  an  ordinary  blue- 
stone  gravity  cell  produces,  under  ordinary 
conditions,  an  E.  M.  F.  of  a  little  more 
than  one  volt.  When  the  terminals  of  a 
single  voltaic   cell  of  this  type   are   con- 

63 


64  ELECTRICITY   IN 

nected  with  a  circuit,  the  electric  current 
which  will  flow  through  the  circuit,  that 
is,  the  amount  of  electricity  which  will 
pass  through  it  per  second,  will  depend 
upon  another  quantity  called  the  resist- 
ance of  the  circuit.  Electric  resistance  is 
that  which  opposes  the  flow  of  electricity 
through  a  circuit. 

Resistance  is  measured  in  units  of  resist- 
ance called  ohms,  after  Dr.  Ohm,  who  first 
pointed  out  the  law  which  governs  the 
flow  of  electricity  through  a  circuit.  The 
resistance  of  a  circuit  depends  upon  its 
length,  its  area  of  cross-section,  and  the 
material  of  which  it  is  composed.  A 
short,  thick  circuit,  of  some  good  conduct- 
ing material  like  copper,  will  have  a  small 
resistance;  a  long,  thin  circuit  of  some 
poor  conducting  material,  like  wet  string, 
will  have  a  high  resistance.     The  greater 


ELECTRO-THERAPEUTICS.  65 

the  length  of  a  conductor,  the  greater 
will  be  its  resistance.  Thus ;  if  we  double 
the  length  of  any  uniform  metallic  wire, 
we  double  its  resistance  ;  or,  if  we  halve 
the  length  of  the  wire,  we  halve  its  resist- 
ance. The  greater  the  area  of  cross-sec- 
tion of  a  conductor  the  less  its  resistance, 
so  that  if  the  area  of  cross-section  of  a 
wire  be  doubled,  retaining  the  same 
length,  its  resistance  will  be  halved. 

The  ohm  is  the  resistance  of  a  definite 
length  of  a  definite  material,  having  a  defi- 
nite area  of  cross-section.  The  ohm  is  de- 
fined as  the  resistance  of  a  column  of  pure 
mercury  having  a  length  of  106.3  cms.,  and 
a  cross-sectional  area  of  one  square  milli- 
metre, at  the  temperature  of  melting  ice, 
or  0°  C.  The  actual  definition  requires, 
not  that  the  cross-sectional  area  should  be 
one  square  millimetre,  but  that  the  weight 


66  ELECTRICITY  IN 

of  the  column  of  mercury  whose  length 
is  106.3  centimetres,  should  be  14.4521 
grammes,  but  this  is  equivalent,  in  a  uni- 
form column,  to  a  cross-section  of  one 
square  millimetre.  In  defining  the  length 
and  the  area  of  cross-section  of  the  column 
of  mercury  which  represents  the  ohm,  it  is 
necessary  to  specify  the  temperature,  since 
the  resistance  of  metallic  bodies  increases 
with  an  increase  in  temperature.  The 
ohm  may  also  be  roughly  stated  as  being 
the  resistance  offered  by  two  miles  of  ordi- 
nary copper  trolley  wire,  or  by  one  foot  of 
copper  wire  No.  40  A.  W.  G.  (American 
Wire  Gauge)  having  a  diameter  of 
0.003145  in.,  at  45°  F. 

In  order  to  compare  the  relative  resist- 
ances of  wires  of  different  materials,  hav- 
ing the  same  length  and  area  of  cross-sec- 
tion,  as  well  as  for  the  purpose  of  being 


ELECTRO-THERAPEUTICS.  67 

able  to  calculate  the  resistance  of  a  given 
length  and  area  of  cross-section  of  any 
material,  it  is  usual  to  consider  the  specific 
resistance  or  resistivity  which  bodies  offer 
to  the  passage  of  electric  currents.  The 
resistivity  of  a  substance  is  numerically 
equal  to  the  resistance  offered  by  a  wire  of 
such  substance  having  unit  length  and 
unit  cross-section.  Any  units  of  length 
and  cross-section  might  be  adopted  for 
this  purpose,  but  the  units  actually 
adopted  are  the  centimetre,  for  the  unit 
of  length,  and  the  square  centimetre  for 
the  unit  of  cross-sectional  area.  The 
resistivity  of  a  substance,  therefore,  is 
numerically  equal  to  the  resistance  offered 
by  a  wire  of  the  substance,  one  centi- 
metre long  and  one  square  centimetre  in 
area  of  cross-section.  In  the  case  of 
metals,  the  resistivity  is  always  a  very 
small  fraction  of  an  ohm,  and  is,  in  fact, 


68 


ELECTRICITY   IN 


usually  expressed  in  microhms;  i.  e.,  in 
millionths  of  an  ohm.  In  the  case  of 
many  liquids,  the  resistivity  is  conven- 
iently expressed  in  ohms,  but  in  the  case 
of  materials  which  possess  very  poor  con- 
ductive power,  generally  called  insulators, 
the  resistivity  becomes  enormously  great 
and  is  more  conveniently  expressed  in 
megohms,  or  millions  of  ohms,  in  begohms, 
or  -billions  of  ohms,  i.  e.,  thousands  of  mil- 
lions, or  in  tregohms,  or  trillions  of  ohms, 
i.  e.y  millions  of  millions. 


The  following  is  a  table  of  resistivities 
of  various  substances : 


Silver,  annealed, 
Copper,      " 
Iron,  " 

Mercury, 
Platinum, 
Pure  water, 
Tap  water,  . 
Hard  rubber, 
Porcelain, 


1.53  microhms,  at  0°  C. 
1.594        "  " 

9.687        "  " 

94.84        "  " 

9.03  "  " 

about  3.75  megohms,  at  10°  C. 
"      200,000  ohms,  at  10°  C. 
.     .     28,000  tregohms. 
.     .     540,000         " 


In  order  to  show  t«.4^e  of  this  table, 
suppose  it  to  be  requirafc<t5  calculate  the 
resistance  of  a  wire  of  platmufeaaje  foot 

long  (30.48  cms.),   and  j^  of  an  inch  in 
diameter ;   i.    e.9   having   a   cross-sectional 

area   of     ~j®     ^^    incn=a0005067 
square  centimetre. 


Looking  at  the  table  of  resistivities  we 

find  for  platinum  the  value  9.03  microhms, 

and  the  resistance  of  the  wire  will,  there- 

.       9.03X30.48  .      , 

fore,  be     a  000^0fi7  ~543,200  microhms = 

0.5432  ohm.  Here  we  multiply  the  resis- 
tivity by  the  length  because  the  longer 
the  wire,  the  greater  will  be  its  resistance. 
If  the  wire  be  30. 48  centimetres  long,  and 
one  square  centimetre  in  area  of  cross- 
section,    it    would    have   a   resistance   of 


70  ELECTRICITY   IN 

9.03X30.48  microhms.  We  divide  by 
the  area,  because  the  greater  the  area,  the 
less  the  resistance.  The  resistance  of  a 
platinum  wire,  one  centimetre  long,  and 
0.0005067  square  centimetre  in  cross-sec- 
tional  area,  would    have   a   resistance    of 

9  03 

QQOQ-Ofi7  =  17,820microhms=0.0l782ohm. 

The  resistance  of  any  metal  can,  the- 
oretically, at  least,  be  computed  in  the 
same  manner,  but  slight  impurities  in  the 
material  are  liable  to  affect  markedly  its 
resistivity,  and  consequently  its  resist- 
ance, so  that,  except  in  the  case  of  very 
nearly  pure  metals,  the  resistances  of  com- 
putation are  not  very  reliable.  The  effect 
of  impurities  is  always  to  increase  the 
resistivity,  and,  therefore,  the  resistance. 

It  will  be  observed  that  most  of  the  re- 
sistivities are   given   at   the    temperature 


ELECTRO-THERAPEUTICS.  71 

0°  C.  Ordinarily,  the  effect  of  temperature 
is  to  increase  the  resistivity  of  all  metallic 
substances.  This  effect  is  nearly  the  same 
for  all  pure  metals,  and  is,  roughly,  4-10ths 
of  one  per  cent,  per  degree  centigrade 
increase  of  temperature  above  zero  centi- 
grade. In  the  case  of  liquids,  and  of  non- 
metallic  substances  generally,  the  resistiv- 
ity diminishes  as  the  temperature  rises. 
Carbon  behaves  in  this  respect  like  an 
insulator,  rather  than  a  conductor.  Tem- 
perature has  a  marked  influence  in  dimin- 
ishing the  resistivity  upon  the  best 
insulators. 

It  may  be  seen  from  the  table,  that  the 
resistivity  of  pure  water  is  very  high  ; 
namely  3.75  megohms,  and  it  is  believed 
by  some,  that  if  water  could  be  obtained 
in  absolute  purity  it  would  not  conduct,  or 
that  its  resistivity  would   be  indefinitely 


72  ELECTRICITY    IN 

great.  Very  slight  degrees  of  impurity- 
suffice,  however,  to  greatly  reduce  the  re- 
sistivity, and  the  addition  of  a  soluble  salt 
reduces  it  to  a  few  ohms.  Thus  the  re- 
sistivity of  a  strong  solution  of  zinc  sul- 


^<^v\aaaaaaaaa^— 


Fig.  26.— Connection  of  Resistances  in  Series. 

phate  in  water  is  about  30  ohms  at  ordi- 
nary temperatures. 

If  a  wire  AB,  of  10  ohms  resistance, 
be  connected  with  a  second  wire  CD,  of 
20  ohms  resistance,  as  shown  in  Fig. 
26,  in  such  a  manner  that  the  current  first 
passes  through  one  and  then  through  the 
other,  they  are  said  to  be  connected  in 
series,  and  the  total  resistance  of  the  series 
AD,  will  be  20+10=30  ohms. 


ELECTROTHERAPEUTICS.  73 

If  two  conductors,  AB  and  CD,  Fig.  27, 

•each  of  10  ohms  resistance,  be  connected  in 

parallel,  as    shown,  so    that    the    current 

divides     between      them,    as      indicated 


// 

'ION 

w 

OF 

(XAA/           x^ 

Fig.  27.- 

—Connect 

Resistances  in 

Parallel. 

by    the   arrows,    the    joint    resistance    of 

10 
the  pair  will  be  -^-=5  ohms.    Similarly,  if 

three  such  wires  be  connected  in  parallel, 
their  joint  resistance  would   be  ~w~  =  3.333 

ohms,  and  so  on,  for  any  number  of  par- 
allel wires. 

The  resistance  of  any  instrument  wound 
with  wire,  such,  for  example,  as  a  tele- 
phone, depends  upon  the  length  and  cross- 


74  ELECTRICITY   IN 

section  of  the  insulated  wire  employed,  as 
well  as  on  the  character  of  the  wire  itself. 
For  a  given  size  of  coil ;  i.  e.,  a  given  vol- 
ume of  winding  space,  the  resistance  in- 
creases rapidly  as  the  diameter  of  the  wire 
employed  to  fill  that  space  is  reduced. 
Neglecting  the  variation  of  the  insulating 
thickness  of  the  wire,  and  its  effects,  the 
resistance  will  increase  inversely  as  the 
fourth  power  of  the  diameter ;  that  is  to 
say,  if  we  halve  the  diameter,  we  shall  in- 
crease the  resistance  of  the  winding  ap- 
proximately 16  times  (24=2x2x2X2)  =  16. 

If  one  Daniell  gravity  cell,  represented 
in  Fig.  28,  be  connected  through  200  feet 
of  No.  25  A.  W.  G.  copper  wire,  to  a  tele- 
phone, the  resistance  of  the  telephone  is 
say  50  ohms,  the  resistance  of  the  wire 
6.5  ohms,  and  the  resistance  of  the  cell, 
say    5  ohms.     The  total  resistance  of   the 


ELECTRO-THERAPEUTICS.  75 

circuit  will,  therefore,  be  50+6.5+5=61.5 
ohms. 

The  electric  resistance  of  any  organic 
substance,  such  as  moist  flesh,  is  very  diffi- 
cult to  determine.  As  soon  as  flesh  is 
dried,  it  becomes  a  non-conductor,  and  it  is, 


Fig.  28.— Resistances  in  a  Circuit  Consisting  op 
a  Voltaic  Cell,  Telephone,  and  Connecting 
Wibes. 

therefore,  evident  that  the  conducting 
properties  of  the  mass  are  due  almost  en- 
tirely to  the  conducting  power  of  the 
liquids  contained  in  it.  These  liquids 
may  be  regarded  as  forming  physically  a 
•continuous  mass,  although,  in  reality,  the 
mass  is  divided  by  numerous  porous  parti- 
tion walls,  or  septa.     The  effect  of  these 


76  ELECTRICITY   IN 

division  walls  is  to  increase   greatly    the 
resistance  offered  by  the  liquids. 

For  the  same  reason,  the  resistance  of 
the  human  body,  or  of  any  portion  of  the 
body,  is  a  very  complex  quantity,  and 
varies  from  time  to  time.  It  also  varies 
with  the  nature  of  the  area  of  the  contact 
surfaces  between  which  the  measurement 
is  taken.  Thus  if  a  bare  copper  wire  be 
grasped  in  each  hand,  the  resistance  of 
the  body,  as  measured  between  the  two 
copper  wires,  may  be  100,000  ohms,  or 
more,  depending  largely  upon  the  dryness 
of  the  skin.  If  now,  instead  of  holding 
the  two  wires,  the  wires  be  connected  to 
metallic  electrodes,  such  as  those  shown 
in  Fig.  29,  the  resistance  apparently 
offered  by  the  body,  as  measured  between  • 
them,  will,  perhaps,  be  only  30,000  ohms, 
for  the  reason   that  the  area  of  cross-sec- 


ELECTRO-THERAPEUTICS.  77 

tion  of  skin,  through  which  the  current 
enters  and  leaves  the  body,  has  been 
greatly  increased;  i.  e.,  the  area  of  cross- 
section  of  the  conductor  in  the  skin  itself 
has  been  greatly   increased.     If  now,  the 


Fig.  29. — Metallic  Handle  Electrodes. 

same  electrodes  be  held  in  the  hands,  while 
wrapped  in  absorbent  cotton,  thoroughly 
wetted  by,  say  salt  water,  the  resistance  of 
the  body,  as  measured  between  them,  will 
be,  perhaps,  5,000  ohms,  owing  to  the  fact 
that  the  skin  has  become  thoroughly  mois- 
tened by  contact  with  the  saline  solution, 


78  ELECTRICITY  IN 

and  has  thereby  become  a  better  con- 
ductor, or  has  had  its  resistance  lowered. 
Again,  if  the  hands  be  dipped  to  the 
wrists  in  jars  containing  salt  and  water,  or 
dilute  caustic  soda,  so  that  the  entire  sur- 
face of  the  hands  is  brought  into  contact 
with  the  liquid,  the  resistance  of  the  body, 
as  measured  between  the  two  jars,  will, 
probably,  be  only  1,000  ohms.  If,  finally, 
the  hands  be  dipped  still  deeper  in  tall 
jars  containing  the  same  solution,  the  re- 
sistance will  still  tend  to  slightly  diminish, 
owing  to  the  greater  area  of  skin  offering 
passage  to  the  current,  and  the  reduced 
effective  length  of  the  circuit  through  the 
body. 

The  principal  resistance  which  the  body 
offers  to  the  passage  of  an  electric  current 
is  that  of  the  skin,  owing,  not  only  to  the 
nature  of  its  substance,  but  also  to  the  fact 


ELECTRO-THEKAPEUTICS.  79 

that  under  ordinary  circumstances  it  is 
dry.  The  strength  of  current,  therefore, 
that  will  pass  through  the  human  body,  in 
the  event  of  an  accidental  contact  with  an 
electric  circuit,  such  for  example  as  a 
trolley  wire,  will  depend  markedly  on  the 
nature  of  the  contact,  as  well  as  on  the 
electromotive  force  in  the  parts  of  the  cir- 
cuit in  contact ;  in  other  words,  the  cur- 
rent received  will  depend  not  only  on  what 
is  touched,  but  also  on  how  it  is  touched. 


CHAPTER  IV. 

ELECTRIC    CURRENT. 

When  we  speak  of  an  electric  current 
flowiug  through  a  circuit  we  mean  the  rate 
at  which  electricity  passes  through  the  cir- 
cuit, and  this  rate  can  be  expressed  by  the 
amount  of  electricity  which  passes  through 
the  circuit  in  a  given  time,  say  in  one 
second,  just  as  the  flow  of  water  through  a 
pipe  can  be  expressed  by  the  quantity  of 
water  which  passes  in  a  given  time.  The 
unit  quantity  of  electricity  is  called  the 
coulomb.  A  little  consideration  will  show 
that  in  the  case  of  liquid  flowing  through 
a  pipe,  since  the  entire  mass  of  the  liquid 
in  the  pipe  is  in  motion,  the  quantity 
which  passes  any  cross-section  of  the  pipe, 

80 


ELECTRO-THlj^PElfTKJS.  r  -.  r^     81  CQ 

in  a  given  time,  mu&t\B&  the  same  as  that  £  j 

which  passes  through  aHjf^tlier  cross-sec-  c  •• 
tion  in  the  same  time,  sure^otneovise, 
there  would  tend  to  be  a  surplus  in  some 
parts  of  the  pipe,  and  a  deficit  in  others. 
The  same  is  true  concerning  an  electric 
flow  or  current.  The  amount  of  electricity 
passing  any  point  of  the  circuit  being 
always  rigorously  equal  to  the  quantity 
which  passes  any  other  point  in  the  same 
time.  We  distinguish  between  the 
quantity  of  electricity  and  the  rate  at 
which  it  flows,  just  as  we  distinguish 
between  a  gallon  of  water,  and  the  rate  of 
flow  of  a  gallon-per-second.  The  electric 
unit  rate  of  flow,  or  unit  of  current  strength, 
is  called  the  ampere,  and  is  equal  to  one 
coulomb-per-second. 

The  ampere  is    the  unit    rate   of  flow 
invariably  employed  in  all  practical  appli- 


82  ELECTRICITY   IN 

cations  of  electricity.  Thus,  an  electric 
incandescent  lamp  may  require  for  its 
operation  a  current  strength  of  from  0.25 
to  10  amperes,  according  to  its  dimensions, 
and  the  amount  of  light  it  is  designed  to 
produce.  In  some  applications  of  elec- 
tricity, as,  for  example,  in  electric  welding, 
or  metal  working,  enormous  currents  have 
to  be  employed,  50,000  amperes  being 
sometimes  required.  In  the  application  of 
electricity  in  electro-therapeutics,  the  cur- 
rent strength  is  always  a  small  fraction  of 
an  ampere,  and  is  generally  measured  in 
milliamperes,  or  thousandths  of  an  ampere, 
for  the  reason  that  a  current  strength  of 
one  ampere  would  be  dangerously  great. 
In  the  application  of  the  death  penalty 
by  electricity,  as  practiced  in  the  State  of 
New  York,  the  alternating  current  strength 
passed  through  the  body  of  the  criminal  is 
usually  7  or  8  amperes. 


ELECTRO-THERAPEUTICS.  83 

When  an  electric  current  is  sent  through 
a  metallic  solution,  such,  for  example,  as 
an  aqueous  solution  of  copper  sulphate,  the 
passage  of  the  current  is  attended  by  a 
decomposition  of  the  salt,  metallic  copper 
being  deposited  on  the  terminal  or  elec- 
trode connected  with  the  negative  pole  of 
the  battery,  and  an  acid  substance  appear- 
ing at  the  electrode  connected  with  the 
positive  pole,  and  entering  into  combina- 
tion with  it.  If  such  combination  be 
chemically  impossible  this  substance  will 
be  liberated  at  the  electrode.  Decompo- 
sition by  electricity  is  called  electrolysis. 
The  amount  of  electrolytic  decomposition, 
that  is,  the  amount  of  saline  substance 
decomposed,  will  depend  upon  the  quantity 
of  electricity  which  passes,  as  well  as  upon 
the  nature  of  the  substance  itself.  An 
ampere  may,  therefore,  be  denned  by  the 
amount  of  chemical  decomposition  which  it 


84  ELECTRICITY   IN 

can  effect  in  a  given  time.  Thus,  since  an 
ampere  is  a  current  of  one  coulomb-per- 
second,  and  each  coulomb  of  electricity 
passing  through  a  solution  of  silver  de- 
posits 1.118  milligrammes  of  silver,  a  cur- 
rent strength  of  one  ampere  produces  a 
deposit  of  1.118  milligrammes  of  silver  per 
second. 

When  the  resistance  of  a  circuit  is 
known  in  ohms,  and  the  electromotive  force 
applied  to  the  circuit  is  known  in  volts, 
the  strength  of  current,  which  passes 
through  the  circuit  in  amperes,  can  be 
readily  calculated  by  a  law  known  as 
Ohm's  law.  Ohm's  law  is  generally  repre- 
sented by  the  equation, 

Volts 

AmPeres=  OW 

Suppose,  for  example,  a  circuit  having  a 
resistance  of  10  ohms  has  an  E.  M.  F.  of 


ELECTRO-THERAPEUTICS.  85 

5    volts   acting   on   it;    then    the  current 

which   flows  through  the  circuit  will  be 

5     1 

—  =-    ampere.      The     practical     electric 

units  of  E.  M.  P.,  resistance  and  current 
strength  may,  therefore,  be  denned,  in 
terms  of  Ohm's  law,  as  follows ; 

A  volt  is  such  a  unit  of  E.  M.  F.  as  will 
produce  a  current  of  one  ampere  in  a  cir- 
cuit whose  electric  resistance  is  one  ohm. 

An  ohm  is  such  a  unit  of  electric  resist- 
ance, as  will  limit  the  flow  of  electricity  to 
a  current  of  one  ampere,  when  under  an 
E.  M.  F.  of  one  volt. 

The  ampere  is  the  rate  of  flow  of  cur- 
rent which  will  pass  through  a  circuit 
whose  resistance  is  one  ohm,  under  an 
E.  M.  F.  of  one  volt. 

The  above  definitions,  although  con- 
venient    for     defining    electric  units    in 


86  ELECTRICITY   IN 

terms   of    one  another,  are   not  generally 
employed. 

The  following  examples  of  the  applica- 
tion of  Ohm's  law  will  show  its  impor- 
tance. Required  the  E.  M.  F.  necessary  to 
produce  a  current  strength  of  10  milliam- 
peres  through  the  human  body,  its  resist- 
ance being,  under  given  conditions,  5,000 
ohms. 

10  mull  amperes  —  i  000=  Too  amPere> 
and  the  E.  M.  F.  which  divided  by  5,000 

ohms  gives  jttt:  =  50  volts,  the  voltage  re- 
quired. 

A  battery  of  fifty  silver-chloride  cells, 
each  having  an  E.  M.  F.  of  1.05  volts,  and 
an  internal  resistance  of  10  ohms  per  cell ; 
it  is  required  to  know  whether  a  single  cell 
or  the  whole  battery  of  fifty  cells  in  series 


ELECTRO-THERAPEUTICS.  87 

will  give  the  greater  current  strength 
through  a  short  stout  piece  of  copper 
wire. 

The  resistance  of  the  short  piece  of  cop- 
per wire  being  negligibly  small  compared 
with  the  resistance  of  a  single  cell,  we  may 
omit  it  altogether  in  the  calculation.  The 
current  strength  from  one  cell  will,  there- 

,      1.05 
fore,  be  -j<7  =  0.105  ampere  —  105  milliam- 

peres.     With  fifty  cells  we  have  50  times 

as  much  E.  M.  F.  and.  also  fifty  times  as 

much  resistance,  and  therefore,  by  Ohm's 

.„  .      52.5 
law,  the  current  strength  will  be  ^tjtj-  = 

0.105  ampere  =  105  milliamperes,  or  the 
same  as  before,  so  that  it  is  evident  that 
through  a  negligibly  small  external  resist- 
ance, or,  as  it  is  called,  on  short  circuit, 
there  is   no  advantage   in   adding  similar 


88  ELECTRICITY   IN 

cells  in  series,  since  although  each  cell  adds 
its  E.  M.  F.  to  the  circuit,  it  also  adds  a 
proportional  amount  of  resistance. 

If  the  silver-chloride  battery  of  the  pre- 
ceding paragraph,  is  employed  to  send  a 
current  through  an  external  resistance  of 
1,000  ohms,  what  will  be  the  current 
strength  with  one  cell  and  with  fifty  cells  ? 

With  one  cell,  the  total  resistance  will  be 
1,000+10  =  1,010  ohms,  and  the  E.  M.  F. 
1.05   volts,  so   that   the   current  strength 

will  be  jTyfA  =  0.00104  =  1.04  milliampere. 

With  fifty  cells,  the  total  resistance  will 

be  1,000+500  =  1,500  ohms,  and  the  E.  M. 

F.    1.05X50  =  52.5    volts.      The    current 

52  5 
strength  will,  therefore,  be ..  5QQ  =  0.035  = 

35  milliamperes. 


ELECTRO-THERAPEUTICS.  89 

As  another  example,  let  us  suppose  that 
two  Gravity-Daniell  cells  have  each  an 
E.  M.  F.  of  1.05  volts,  and  a  resistance  of 
4  ohms.  Find  the  current  strength  which 
they  can  send  through  a  resistance  of  one 
ohm  externally;  (a)  When  connected 
singly  ;    (b)  In  series ;     (c)  In  parallel. 

1.05      1.05 
(a)  jrrr  =  ^-  =  0.21  =  0.21   ampere  = 

210  milliamperes. 

1.05X2       2.1 
(J)  In  series,  ^^+1  =  ~9~  =  °-233  = 

233  milliamperes. 

(c)  In  parallel.  Here  the  positive  pole 
of  one  cell  is  connected  to  the  positive  pole 
of  the  other,  and  the  negative  pole  of  one 
cell,  connected  to  the  negative  pole  of  the 
other.  The  E.  M.  F.  of  the  combination 
will  be  that  of  a  single  cell,  but  the  resist- 
ance of  the  combination  will  be  that  of 
two  equal  resistances  in  parallel,  or  one 


90  ELECTRICITY  IN 

half  that  of  either;   namely,  2  ohms,  the 

current  strength  will,  therefore,  be  ^xr  = 

1.05 

— Q-  =0.35  ampere  =  350  milhamperes. 


An  instrument  for  measuring  the 
strength  of  electric  currents  is  called  an 
amperemeter,  or  ammeter.  For  electro- 
therapeutic  purposes,  since  the  current 
strength  to  be  measured  is  usually  ex- 
pressed in  milliamperes,  the  instrument  is 
frequently  called  a  milliammeter. 

Milliammeters  are  made  in  a  variety  of 
forms.  In  nearly  all  cases,  however,  an 
index  or  pointer  is  moved  over  a  graduated 
scale  by  the  force  exerted  between  a  coil 
of  wire  carrying  the  current  to  be  meas- 
ured, and  a  magnet  in  its  vicinity.  This 
movement  is  due  to  the  fact  that  a  wire 


&  DfESR 


m 


ELECTRO-THERAPBI^ICS/    Kl   »^BllTV     Lf 


carrying  an  electric  currentN&^mes  tem- 
porarily invested  with  magnetic 


*fffe®|$    ! 


Fig.  30.— Form  of  Milliammetek. 

A  simple  form  of  milliammeter  is  shown 
in  Fig.  30.  In  this  a  pair  of  coils  of  wire, 
situated  beneath  the  horizontal  face  of  the 
instrument,  become  magnetized  by  the 
passage  of  the  current  to  be  measured,  and 


92  ELECTRICITY   IN 

deflect  a  magnetic  needle,  in  the  shape  of 
a  split  bell,  from  the  position  it  assumes 
under  the  influence  of  the  earth's  field  at 
the  place  where  the  measurement  is  made. 


Fig.  31. — Vertical  Form  of  Milliammeter. 

The   scale    is    marked    directly   in    milli- 
amperes. 

Another  form  of  instrument  is  shown  in 
Fio*.   31.     Here  a  vertical    needle    is    de- 


ELECTRO-THERAPEUTICS.  93 

fleeted  by  the  attraction  of  a  coil  of  wire 
upon  a  magnetized  needle  placed  inside 
the  instrument. 


Fig.  32. — Form  of  Milliammeter. 

Still  another  form  of  instrument,  differ- 
ing only  in  constructive  details  from  those 
above  described,  is  shown  in  Fig.  32. 

While  the  preceding  instruments  are 
simple  in  their  construction,  yet  they  are 
all  liable  to  have  their  indications  attended 


94 


ELECTRICITY   IN 


by  changes  of  magnetic  force  occurring  in 
their  neighborhood,  and  even  when  all 
magnets  are  carefully  removed  from  their 
vicinity,    their    indications    are    often   at- 


Fig.  33.— Weston  Milli ammeter. 


tended  by  appreciable  error,  due  to  some 
difference  between  the  strength  of  the 
earth's  magnetic  force  at  the  place  where 
the  instrument  is  employed,  and  the  place 
where  it  was  originally  calibrated. 


ELECTRO-THERAPEUTICS. 


95 


A  form  of  instrument  which  is  practi- 
cally free  from  the  earth's  magnet  inilu- 


Fia.  34. — Working  Parts  of  Weston  Ammeter. 


ence  is  shown  in  Fig  33.     This  freedom  is 
owing  to  the  fact  that  the  instrument  has 


96  ELECTRICITY   IN 

no  iron  moving  parts.  The  principal 
working  parts  are  shown  in  Fig.  34.  A 
horse-shoe  permanent  magnet  MM,  MM, 
only  partly  visible  in  the  figure,  has  soft 
iron  projections,  or  pole-pieces  P,  P,  secured 
to  its  poles.  These  projections  are  shaped 
so  as  to  enclose  a  cylindrical  space.  At 
the  centre  of  this  space  is  supported  a  soft 
iron,  solid  cylindrical  core  I  I.  Between 
this  core  and  the  pole-j>ieces,  there  remains 
a  narrow  annular  gap,  or  space,  which  is 
permeated  by  the  magnetic  flux  from  the 
permanent  magnet.  In  this  space  a  deli- 
cately supported  coil  C,  of  insulated 
copper  wire,  is  free  to  move.  Coil  springs 
S,  S,  carry  the  current  to  be  measured  into 
and  out  of  the  coil  C.  So  long  as  there  is 
no  current  through  the  coil,  it  is  unaffected 
by  the  magnetic  field  in  which  it  is  placed, 
and  the  pointer  remains  at  the  zero  point. 
When,  however,  a  current  passes  through 


ELECTRO-THERAPEUTICS.  97 

the  coil,  the  electromagnetic  action  of  this 
current  upon  the  magnetic  field,  causes 
a  mechanical  force  to  be  exerted  upon 
the  coil,  deflecting  it  against  the  spiral 
springs  S,  S}  in  such  a  manner  that  the 
pointer  JS,  moves  over  a  scale  beneath  it 
through  a  distance,  which  is  almost  di- 
rectly porportional  to  the  current  strength. 
The  permanent  magnet's  field  is  so 
very  much  more  powerful  than  the  earth's 
magnetic  field,  that  the  influence  of  the 
latter  upon  the  coil  is  negligible  by  com- 
parison. The  accuracy  ojf  the  instrument 
depends  upon  the  degree  of  permanence 
with  which  the  magnetism  in  the  horse- 
shoe magnet  is  retained.  The  instrument 
has  the  disadvantage  that  it  will  not  read 
in  either  direction,  so  that  if  the  current 
passing  through  the  instrument  has  the 
wrong  direction,  the  wires  attached  to  the 
instrument  must  be  reversed. 


98  ELECTRICITY   IN 

In  several  of  the  instruments  just 
described,  means  are  provided  for  varying 
their  sensibility  and  the  range  of  their 
indications.  Thus,  in  Figs.  29  and  30, 
a  screw  button  is  seen  projecting  from  the 
face  of  the  apparatus,  marked  10.  If  this 
screw  be  pressed  forward,  until  it  abuts 
strongly  against  its  stop,  a  coil  of  wire  will 
be  brought  into  connection  with  the  termi- 
nals, in  such  a  manner  that  9-10ths  of  the 
current  passing  through  the  instrument 
will  pass  through  this  wire,  and  only 
1-1 0th  will  pass  through  the  measuring 
coils.  The  instrument  under  these  con- 
ditions is  said  to  be  shunted,  or  to  have  a 
shunt  applied  to  it  whose  power  is  10. 
The  readings  of  the  instrument  in  milliam- 
peres  must  now  be  multiplied  by  10,  in 
order  to  obtain  the  actual  current 
strengths.  In  Fig.  33,  three  terminals  are 
shown   and    the   instrument    is    provided 


ELECTRO-THERAPEUTICS.  99 

with  two  sets  of  graduations  on  its  scale. 
When  the  right  hand  and  the  corner  left 
hand  terminals  are  used,  the  lower  scale  is 
brought  into  use,  by  which  the  instrument 
reads  up  to  10  milliamperes  only,  but  by 
using  the  right  hand,  and  the  inner  left 
hand  terminals,  the  upper  scale  is  utilized, 
by  which  currents  up  to  500  milliamperes 
can  be  measured. 

Instruments  of  the  preceding  types  are 
sufficiently  sensitive  for  all  the  ordinary 
requirements  of  electro-therapeutic  applica- 
tions. When,  however,  physiological  re- 
searches have  to  be  made,  in  which  very 
feeble  electric  currents  are  measured,  it  is 
necessary  to  use  a  mirror  galvanometer,  as 
a  form  of  ammeter.  Such  an  instrument 
is  made  in  a  variety  of  forms,  one  of  the 
simplest  of  which  is  illustrated  in  Fig.  35. 
A  circular  coil  or  spool  of  fine  insulated 


100 


ELECTRICITY   IN 


copper   wire   is   mounted   upon   a   tripod 
frame,  and    a    small    magnetized    needle 


Fig.  35.— Mirror  Galvanometer 


is  suspended  by  a  fibre  of  silk  at  the 
centre  of  the  coil.  A  small  glass  mirror 
is   attached    to    the   suspension    in    such 


ELECTRO-THERAPEUTICS. 


101 


a  manner  that  any  deflection  of  the 
little  magnetized  needle  will  cause  an 
angular  deflection  of  the  mirror.  Facing 
the  instrument  is  a  lamp  and  scale,  shown 
in    Fig.    36.      The     lamp,    whose     glass 


Fig.  36.— Mirrok  Galvanometer  Scale. 


chimney  only  is  seen  at  G>  throws  a 
beam  of  light  through  the  window  W,  on 
the  mirror  suspended  in  the  galvanometer. 
The  latter  reflects  the  beam  on  the  scale  S. 
Since  a  deflection  of  the  mirror  through  an 
angle  of  45°,  would  be  sufficient  to  deflect 


102  ELECTRICITY   IN 

the  beam  through  a  right  angle,  or  90°, 
and,  therefore,  to  send  the  beam  to  the  end 
of  an  indefinitely  long  scale,  it  is  evident, 
that  a  veiy  small  angular  deviation  of  the 
rnaomet  under  the  influence  of  the  current 
to  be  measured  passing  through  the  coil, 
will  suffice  to  produce  a  marked  displace- 
ment of  the  spot  of  light  along  the  scale  S. 
A  current  in  one  direction  will  deflect  the 
beam  to  the  right,  and  a  current  in  the 
opposite  direction,  to  the  left.  Such  an 
apparatus  has  commonly  a  resistance  of 
500  ohms,  and  a  current  of  one  millionth 
of  an  ampere,  on  one  micro-ampere,  will 
deflect  the  beam  through  a  distance  of  a 
millimetre  on  a  scale  one  metre  distant. 

In  researches  of  very  great  delicacy, 
where  exceedingly  feeble  currents  have  to 
be  observed,  special  very  sensitive  mirror 
galvanometers  are  employed.     One  of  these 


ELECTRO-THERAPEUTICS. 


103 


is  shown  in  Fig.  37.     Here  four  sets  of  coils, 
one  above  another,  act  on  four  little  mag- 


•""■TlUffll:" 
Fig.  37.— Sensitive  Mirror  Galvanometer. 


netic  needles  situated  at  their  respective 
centres.  A  single  mirror,  attached  to  the 
upper   part  of  the  suspension,  reflects  its 


104  ELECTRICITY   IN 

beam  of  light  through  the  window  W. 
The  terminals  of  the  coils  are  brought  to 
the  connecting  posts  t,  t.  m,  tn,  are  two  con- 
trolling magnets  employed  for  bringing  the 
magnetic  needles  back  to  the  same  posi- 
tion after  the  application  of  the  current 
to  be  measured.  Such  an  instrument  has 
a  resistance  of  about  15,000  ohms,  and  has 
a  sensibility  such  that  one  billionth  of  an 
ampere,  or  one  bicro-ampere,  i.  e.  one 
thousand  millionth  ampere,  will  produce  a 
deflection  of  15  millimetres  on  a  scale  dis- 
tant one  metre.  Except  for  delicate  work, 
and  very  feeble  currents,  the  Thomson 
galvanometer  is  undesirable,  as  the  values 
of  its  indications  have  usually  to  be  con- 
verted into  amperes  by  careful  measure- 
ments and  computations. 

A     form   of    galvauometer,    very   con- 
venient when  the  greatest  sensibility  is  not 


"<«?CF^?ry  Cf " 

required,  is  shown  in  Fig^&S-^and  is  called 
the    D'Arsonval    galvanonn 


ELECTRO-THERA»^Jr 


Fig.  38.— D'Arsonval  Galvanometer. 


coil  of  insulated  wire  C,  is  suspended 
between  the  poles  of  a  permanent  magnet 
M,  and  by  means  of  the  attached  mirror, 


106  ELECTRICITY. 

the  deflection  of  this  coil  can  be  observed. 
It  is  evident  that  while  in  the  preceding 
mirror  galvanometers,  the  coil  is  fixed 
and  the  magnet  is  movable,  in  this  in- 
strument the  magnet  is  fixed  and  the  coil 
is  movable. 


CHAPTER  V. 

VARIETIES    OF   ELECTROMOTIVE    FORCE. 

The  voltaic  or  primary  cell,  and  the 
secondary  cell  already  described,  will  pro- 
duce an  E.  M.  F.  which,  so  long  as  the 
chemicals  remain  unchanged,  does  not  vary 
in  strength.  Such  an  E.  M.  F.  is,  there- 
fore, called  a  continuous  E.M.F.  A  con- 
tinuous E.  M.  F.  is  also  produced  by  a 
variety  of  other  electric  sources,  such,  for 
example,  as  a  continuous-current  dynamo, 
which,  so  long  as  its  speed  of  rotation 
remains  the  same,  produces  an  E.  M.  F. 
which  is  practically  continuous. 

Fig.  39,   represents   graphically   a   con- 
tinuous E.  M.  F.     The  straight  line  AB,  is 
-   107 


108 


ELECTRICITY   IN" 


drawn  parallel  to  the  base  OS,  at  a  dis- 
tance representing  1.1  volts.  Time  is 
measured  along  the  base  OS,  and  the  fact 
that  the  line  AB,  remains  parallel  to  the 


> 

2.2C 


1.1  A 

0 

-LIE 


■f S 


Fig.  39.— Continuous  E.  M.  F. 


base,  represents  the  constancy  of  the 
E.  M.  F.,  which  might  be  that  of  a  single 
Daniell  cell.  Two  such  cells,  connected 
in  series,  would  produce  a  continuous 
E.  M.  F.  of  2.2  volts,  represented  by  the 
straight  line  CD,  twice  as  far  above  the 
line  OS,  as  the  line  AB. 


ELECTRO-THERAPEUTICS.  109 

An  E.  M.  F.  possesses  direction,  as  well 
as  magnitude;  that  is  to  say,  it  may  tend 
to  send  a  current  through  a  circuit  in  one 
direction  or  in  the  opposite  direction.     All 

E.  M.  F.'s  that  tend  to  send  the  current  in 
one  direction  may  be  regarded  as  positive, 
and  all  tending  to  send  the  current  in  the 
opposite  direction,  as  negative.      Positive 

F.  M.  Fh  are  represented  graphically  by 
distances  above  the  line  OS,  and  negative 
F.  M.  F.'s,  by  distances  below.  Thus,  in 
Fig.  39,  the  line  FF,  would  indicate  a 
negative  E.  M.  F.  of  1.1  volts,  or  an 
E.  M.  F.  oppositely  directed  to  that  of 
the  line  AB. 

Fig.  40,  shows  the  E.  M.  F.  produced  by 
a  continuous-current  dynamo.  Here  the 
line  AB,  is  parallel  to  the  base  as  before, 
but  instead  of  being  straight,  is  a  fine, 
wavy  line.     These   little  waves  represent 


110 


ELECTRICITY    IN 


variations  in  the  amount  of  E.  M.  F.  pro- 
duced every  time  that  the  bar  in  the  com- 
mutator passes  underneath  the  collecting 
brush.  These  wavelets  exist  in  the 
E.   M.  F.     of     every     continuous-current 


IIS 

mi 
111 

CO 

H10 
£l09 


m 


A  B 


n 


T 


0  * 

SECONDS 

Fig.  40. — Type  of  E.  M.  F.  Produced  by  a  Contin- 
uous-Current Dynamo. 


dynamo.  When  they  are  very  marked, 
as  represented  in  Fig.  41,  the  E.  M. 
F.  is  said  to  be  pulsatory.  Such  E. 
M.  F.'s  are  produced  by  some  con- 
tinuous-current   generators,     usually      for 


ELECTRO-THERAPEUTICS. 


Ill 


supplying  arc  lamps.  It  is  evident,  that 
at  different  times  the  E.  M.  F.  varies  con- 
siderably in  its  magnitude,  but  never 
changes  direction,  the  line  AB,  being 
always  on  one  side  of  the  zero  line  08; 
that  is  to  say,  it  always  has  the  same  direc- 


±      A       L 
10        10         2 

SECONDS 


Fig.  41.— Pulsatory  E.  M.  F. 


tion  in  the  circuit,  just  as  though  a  battery 
of  voltaic  cells  were  employed  to  send  cur- 
rent through  a  circuit,  and  that  at  inter- 
vals, a  certain  number  of  these  cells  were 
cut  out  and  re-introduced. 


112 


ELECTRICITY   IN 


When  the  waves  start  each  time  from 
the  zero  line,  the  E.  M.  F.  is  said  to  be 
intermittent.  Fig.  42,  shows  that,  at  cer- 
tain intervals,  an  E.  M.  F.  exists  in  the  cir- 
cuit in  one  direction,  and  that  at  interven- 


P  TIME 

Fig.  42.— Intermittent  E.  M.  F.    Undirectional. 

ing  intervals  there  is  no  E.  M.  F.  The 
intermittent  E.  M.  F.  can  be  obtained  by 
connecting  a  continuous  E.  M.  F.,  say  a 
voltaic  battery,  to  a  wheel  interrupter,  in 
such  a  manner  that  the  E.  M.  F.  will  be 
periodically  cut  off  and  applied.  In  all 
cases,  although  the  strength  of  the 
E.  M.  F.  varies  at  different  times,  yet  at  no 


ELECTRO-THERAPEUTICS. 


113 


time  does  it  change  direction,  so  that  the 
curved  line  lies  wholly  above  the  base 
line.  When  an  E.  M.  F.  changes  direc- 
tion,   as    well    as   magnitude,  it    becomes 


4-10— 

4-5  -m 

CO 

i- 
_i 

3 

£  0    . 

SECOKOS 

B 

-  -5-£ 

t 

-10- 

A 

Fig.  43.— Alternating  E.  M.  F. 


alternating.  Thus,  in  Fig.  43,  the 
E.  M.  F.  is  seen  to  alternate  between  10 
volts  positive  and  10  volts  negative,  the 
transitions  in  this  particular  case  being 
made  instantaneously.  Such  an  E.  M.  F. 
might  be  produced  by  connecting  a 
battery    of    .voltaic  cells   with   a   current 


114  ELECTRICITY   IN 

reverser,  in  such  a  manner,  that  by  rotat- 
ing the  handle,  the  E.  M.  F.  would  be 
periodically  reversed  without  being  with- 
drawn from  the  circuit. 


4100- 

t  80- 

A 

4  60- 

4  40- 

tt+  20- 

7i      n 

\B 

D/ 

o      O 
-.20- 

SECONDS 

1  \ 

/ 2 

/lOO 

-  40- 

-60- 

-  80- 

>s*^iL^ 

-100- 

Fig.  44.— Symmetrical  Alternating  E.  M.  F. 

It  is  not  necessary  that  an  alternating  E. 
M.  F.  should  change  abruptly  from  its 
maximum  positive  to  its  maximum  nega- 
tive value.  In  most  cases,  in  fact,  the 
change  occurs  more  gradually,  as  shown 
in   Fig.    44,    which   represents  a  common 


# 


^W?  &  DIESEL  ^ 


'¥, 


ELECTRO-THERJ 


type  of  alternating  E.  M.  l^'^Eigs.  45  and 
46,  represent  the  same  alternating  E.  M. 
F.,  although  the  graphical  appearance  of 
the  waves  is  changed,  owing  to  the  varia- 


4100- 

A 

80- 

£  60- 

-J   40- 

9.   20- 

\b 

d/ 

U 
-20- 

SECONDS 

106\ 

-  40- 

-  60- 

-  80- 

^-— Q— ^ 

-100- 

Fig.  45.— Symmetrical  Alternating  E.  M.  F. 


tions  of  the  scale  of  time  along  the  base,  and 
the  scale  of  E.  M.  F.  alon^  the  vertical.  It 
will  be  observed  that  in  all  representations 
of  alternating  E.  M.  F.  there  is  a  motion  in 
one  direction,  in  which  the  E.  M.  F.,  begin- 
ning at  the  base  line  or  zero,  gradually  in- 
creases in  value,  and  then  gradually  falls 


116 


ELECTRICITY   IN 


until  it  again  reaches  zero,  then  changing 
its  direction  and  going  through  the  same 


100— 

A 

80- 

/\ 

VOLTS 

1         1 

\          1 

20— 

/                               \                                  / 

1                                   \B                              D/         „ 

0 

SECONDS                                                            I             ° 

-111                  "If 

"-20- 

1                     I 

-40- 

\                  / 

-60- 

\               / 

-80- 

\<LS 

,-100- 

G.  46.- 

—Symmetrical  Alternating  E.  M.  F. 

changes  in  the  opposite  direction.  Thus 
the  E.  M.  F.  commencing,  in  Fig.  44,  at 
zero,  advances  in  the  positive  direction  to 


ELECTRO-THERAPEUTICS.  117 

its  maximum  or  greatest  value  at  A,  then 
diminishing,  but  still  in  the  positive  direc- 
tion, to  zero,  at  B,  changing-  direction  and 
increasing  negatively  to  a  maximum  value 
at  C,  and  then  regularly  decreasing  again 
to  zero  where  it  again  repeats  the  former 
movement.  Each  of  the  waves  OAB,  or 
BOB ,  is  called  an  alternation ;  so  that  it 
will  be  seen,  that  in  the  cases  considered 
in  Figs.  44,  45,  and  46,  an  alternation  lasts 
l/100th  of  a  second,  or,  there  are  100  alter- 
nations in  a  second.  A  complete  to-and- 
fro  motion  is  called  a  cycle,  and  the  time 
required  to  complete  a  cycle  is  called  a 
period.  The  period  in  the  cases  here  con- 
sidered is  l/50th  second  and  it  is  evident 
that  the  number  of  cycles  in  a  second 
will  depend  upon  the  value  of  the  period. 
For  example,*  if  the  period  is  1/1 00th  of 
a  second,  there  would  then  be  100  cycles 
in  a  second.     The  number  of  cycles  in  a 


118 


ELECTRICITY  IN 


second  is  called  the  frequency  ;  so  that,  in 
the  last  case  the  frequency  would  be  100. 
This  is  often  written  100~. 


Fig.  47.— Symmetrical  Alternating  E.  M.  F. 


Alternating  E.  M.  F.'s  may  be  sym- 
metrical, or  dissymmetrical.  A  symmetri- 
cal E.  M.  F.  is  one  which  is  graphically 
symmetrical  about  its  zero  ljne;  that  is 
to  say,  the  positive  waves  are  the  same  as 
the  negative  waves,  except  that  they  occur 
in  the  opposite  direction.     Fig.  47,  repre- 


ELECTRO-THERAPEUTICS.  119 

sents  a  type  of  symmetrical  wave  of  E.  M. 
F.  having  a  frequency  of  75  ~  ;  and,  conse- 
quently, a  period  of  1/7 5th  second.  A  dis- 
symmetrical alternating  E.  M.  F.  is  one  in 


Fig.  48.— Dissymmetrical  Alternating  E.  M.  F.     m 

which  the  positive  wave  differs  from  the 
negative  wave  not  merely  in  direction  but 
also  in  outline.  A  type  of  dissymmetrical 
wave  is  shown  in  Fig.  48. 

Symmetrical  alternating  E.  M.  F.  waves 
are  produced  by  alternating -current  dyna- 
mos, or  alternators.     Dissymmetrical  alter- 


120  ELECTRICITY   IN 

nating  E.  M.  F.  waves  are  j)roduced  by 
particular  types  of  apparatus,  such  s&fara- 
dic  coils,  the  construction  of  which  will  be 
explained  hereafter. 


G 

Fig.  49.— Sinusoidal  E.  M.  F. 


It  is  evident  from  the  preceding  figures 
that  an  E.  M.  F.  becomes  alternating  if 
it  periodically  changes  its  direction  and 
magnitude,  and  that  considerable  varia- 
tion may  exist  in  the  manner  in  which 
both  of  these  changes  may  occur.  A 
wave  of  the   form   shown  in  Fig.   49,  is 


ELECTRO-THERAPEUTICS.  121 

called  a  sinusoidal  wave,  and  an  E.  M.  F. 
alternating  in  this  manner  is  called  a  sinu- 
soidal R  M.  F.  This  type  of  E.  M.  F. 
possesses  important  characteristics. 

It  is  evident  that  the  following  distinct 
types  of  E.  M.  F.  exist ;  namely, 

(  Pulsating  i  Intermittent, 
f  Continuous  \  <  Non-Intermittent. 

(  Steady 

(Symmetrical...  |  Sinusoidal. 
[Alternating-^  <  Non-sinusoidal. 

(  Dissymmetrical 

A  continuous  E.  M.  F.  acting  in  a  closed 
circuit  produces  in  it  a  continuous  current. 
A  pulsating  E.  M.  F.  similarly  produces  a 
pulsating  current.  An'  alternating  E.  M. 
F.  produces  an  alternating  current. 

•  In  fact,  all  the  varieties  of  E.  M.  F.  in 
the  above  table,  when  acting  in  a  circuit, 


122  ELECTRICITY. 

produce  their  particular  type  of  electric 
current,  although  the  graphic  representa- 
tion, of  the  current  is  not  always  the 
exact  counterpart  of  the  graphic  repre- 
sentation of  the  E.  M.  F.  The  above 
table  may,  therefore,  be  repeated  for  cur- 
rents as  well  as  for  E.  M.  F.'s. 

The  character  of  the  electric  current  is 
dependent  on  the  character  of  the  E.  M.  F. 
producing  it.  A  continuous  electric  cur- 
rent, like  the  continuous  E.  M.  F.  which 
causes  it  to  flow,  does  not  vary  in  its 
strength  at  different  times,  but  flows 
through  the  circuit  like  a  steady  flow  of 
water  through  a  pipe  or  river  channel.  A 
continuous  electric  current  is  sometimes 
called  a  direct  current,  in  contradistinc- 
tion to  an  alternating  electric  current, 
which,  like  the  E.  M.  F.  producing  it," 
changes  its  direction  at  every  half  cycle. 


ELECTRO-THERAPEUTICS.  123 

All  electric  currents,  however,  are  produced 
by  the  action  of  some  form  of  E.  M.  R,  so 
that  the  presence  of  current  in  any  circuit 
necessitates  the  existence  of  an  E.  M.  F. 
which  has  produced  it. 


CHAPTER  VI. 

ELECTRIC    WORK    AND    ACTIVITY. 

An  electric  current  is  never  produced 
Avithout  an  expenditure  of  work.  The 
greater  the  strength  of  the  current,  the 
greater  is  the  amount  of  work  done  in  a 
given  time.  In  mechanics,  the  amount  of 
work  done  by  the  action  of  any  force  is 
measured  by  the  distance  through  which 
the  force  acts.  A  convenient  unit  of  work 
is  the  foot-pound,  or  the  amount  of  work 
done  when  one  pound  is  raised,  against  the 
force  of  gravity,  through  a  vertical  dis- 
tance of  one  foot.  Since  to  do  work 
requires  the  expenditure  of  energy,  the 
rate   at   which   work   is   done   represents 

124 


ELECTRO-THERAPEUTICS.  125 

the  rate  at  which  energy  is  expended. 
Thus,  a  pound  weight  raised  through  a 
foot,  requires,  as  we  have  seen,  the  expen- 
diture of  a  foot-pound  of  work.  The  same 
amount  of  work  would  be  done  if  a  pound 
weight  were  raised  through  a  foot  in  a 
minute,  or  in  a  second,  but  the  rate  at 
which  energy  would  be  expended  would 
be  sixty  times  greater  in  the  second  case 
than  in  the  first. 

The  rate  of  doing  work,  or  expending 
energy,  is  called  activity.  For  the  electric 
unit  of  work  the  foot-pound  might  be 
employed,  but  it  is  more  convenient  in 
practice  to  employ  a  unit  called  the  joule. 
The  joule  is  nearly  equal  to  0.738  foot- 
pound ;  that  is  to  say,  one  foot-pound  is  an 
expenditure  of  work  equal  to  about  1.355 
joules.  In  the  same  way  the  foot-pound- 
per-second,  the  mechanical  unit  of  activity, 


126  ELECTRICITY   IN 

might  be  employed  as  the  electric  unit 
of  activity,  but  it  is  more  convenient  to 
employ  the  joule-per-second,  or  the  watt, 
of  which  746  are  equal  to  one  horse-power, 
of  550  foot-pounds  per  second. 

An  electromotive  force  or  pressure  is 
always  required  to  send  a  current  through 
a  circuit ;  that  is  to  cause  electricity  to 
flow,  and,  as  in  the  case  of  mechanical 
work,  the  amount  of  electrical  work  done 
is  equal  to  the  amount  of  electricity  set  in 
motion  multiplied  by  the  pressure  which 
causes  it  to  move.  When  a  unit  of  E.  M. 
R,  or  one  volt,  causes  one  coulomb  of  elec- 
tricity to  pass  through  a  circuit,  there  is 
one  unit  of  work  expended  called  the 
volt-coulomb,  or  the  joule.  When  a  pres- 
sure of  one  volt  causes  a  coulomb-per- 
second  to  pass  through  the  circuit,  that  is 
when  a  volt  causes  a  current  of  one  ampere 


to  flow,  electric  work  is  abrffein  the  cir-  * 

cuit  at  the  rate  of  one  watt,  so  that  if  we 
multiply  the  volts  in  the  circuit  by  the 
amperes,  we  have  the  activity  in  the  circuit 
expressed  in  watts.  Thus,  if  an  electric 
pressure  of  20  volts,  measured  between 
electrodes,  be  applied  to  the  human  body, 
and  the  current  produced  in  the  circuit 
under  these  conditions  be  25  milliamperes, 
then  the  electric  activity  in  the  body  will 

be  20xifro  =  wo =  °-5  watt' or  half  a 

joule  each  second;  i.  e.y  0.369  foot-pound 
each  second. 

If  a  galvano-cautery  be  supplied  with  a 
current  of  20  amperes,  under  a  pressure  of 
2  volts,  the  activity  in  the  knife  will  be 
2  x  20  =  40  watts  =  40  joules-per-second  = 
29.52  foot-pounds-per-second  ;  and,  if  this 
activity  be  sustained  for  two  minutes,  the 


128  ELECTRICITY   IN 

work  done  will  amount  to  40  X 120  =  4,800 
joules  =  3,546  foot-pounds;  that  is  to  say 
to  1  pound  lifted  3,546  feet,  or  to  one  ton 
of  2,000  pounds  lifted  1.723  feet. 

The  amount  of  activity  expended  in  a 
small  incandescent  lamp,  of  say  1/2  candle 
power,  frequently  employed  for  exploring 
cavities  in  the  human  body,  may  be  found 
from  the  fact  that  such  a  lamp  only  re- 
quires a  current  of  1.4  amperes,  at  a  pres- 
sure of  3  volts  at  its  terminals.  This 
represents  an  activity  of  3  X  1.4  =  4.2 
watts,  or  an  expenditure  of  8.4  watts  per 
candle. 

Electric  activity  in  a  circuit  has  al- 
ways to  be  provided  by  the  source  of  the 
driving  E.  M.  F.  Thus,  when  a  voltaic 
battery  is  supplying  a  pressure  which 
drives  a  current,  it  is  the  chemical  energy 


ELECTRO-THERAPEUTICS.  129 

in  the  battery  which  has  to  provide  the 
activity  and  the  work  done.  In  other 
cases,  where  a  dynamo    is  the  source  of 

E.  M.  P.,  the  power  has  to  be  supplied 
from  the  engine  which  drives  the  dynamo. 
When,  therefore,  an  E.  M.  F.  drives  a  cur- 
rent, it  does  work  on  that  current,  and  the 
work  must  be  supplied  by  the  source  of  E. 
M.  F.     On  the  other  hand,  when  an  E.  M. 

F.  is  driven  by  a  current,  that  is  to  say 
when  a  current  passes  in  a  circuit  against 
the  action  of  an  E.  M.  F.,  so  that  the  cur- 
rent overcomes  the  E.  M.  F.,  then  work  is 
done  upon  the  E.  M.  F.  instead  of  by  the 
E.  M.  F.,  and  work  appears  in  the  source 
of  E.  M.  F.  For  example,  when  a  current 
passes  through  a  voltameter  ;  i.  e.,  a  vessel 
containing  acidulated  water,  and  provided 
with  platinum  electrodes,  an  E.  M.  F.  is 
set  up  at  the  surface  of  the  immersed 
electrodes,  opposing  the  passage  of  the  cur- 


130  ELECTRICITY  IN 

rent.     Such  an  E.  M.  F.  is  called  a  counter 
M  M.  F.,  and  is  abbreviated  C.  E.  M.  P. 

Fig.  50,  represents  a  form  of  voltameter. 
The  current  passes  between  the  binding 
posts  through  the  acidulated  water  con- 
tained in  the  vessel  A,  being  led  in  and 
out  by  platinum  plates  or  electrodes.  If  a 
current  of  two  amperes  passes  through 
such  an  apparatus,  and  a  C.  E.  M.  F.  of 
2.5  volts  be  set  up  at  the  surface  of  the 
electrodes,  an  activity  will  be  expended  of 
2.5X2=5  watts,  upon  this  E.  M.  F.,  and 
this  work  will  be  expended  chemically, 
in  decomposing  the  acidulated  water  and 
liberating  its  constituent  gases,  oxygen  and 
hydrogen,  which  appear  at  the  terminals 
connected  respectively  with  the  positive 
and  negative  poles  of  the  battery.  As- 
suming that  none  of  the  liberated  gas 
is   dissolved,   it    will    accumulate    in   the 


ELECTRO-THERAPEUTICS. 


131 


Fig.  50. 


collection  tubes  over  the  respective  elec- 
trodes, and,  if  allowed  to  enter  into  com- 
bination at  any  subsequent  period,  will 
liberate  an   amount   of   work   in   the   ex- 


132  ELECTRICITY   IN 

plosion,  equal  to  the  work  done  by  the 
electric  current  in  evolving  it.  A  volta- 
meter is  used  for  measuring  the  strength 
of  a  current  by  the  rate  of  decomposition 
of  water.  It  is  not,  however,  as  convenient 
for  such  purposes  as  an  ammeter. 

When  an  electric  current  passes  through 
a  wire  offering  a  resistance,  a  C.  E.  M.  F.  is 
practically  developed  in  the  wire ;  that  is 
to  say,  if  a  current  of  5  amperes  passes 
through  a  resistance  of  2  ohms,  a  C.  E.  M. 
F.  of  10  volts  will  be  established  in  the 
wire,  for  the  reason  that,  by  Ohm's  law,  10 
volts  are  necessary  at  the  terminals  of  the 
wire  in  order  to  send  five  amperes  through 
2  ohms  resistance.  The  product  of  the  C. 
E.  M.  F.  and  the  current,  represents  the  ac- 
tivity expended  on  the  C.  E.  M.  F.,  and  this 
work  is  always  expended  in  heating  the 
wire.      In    the    case   assumed,    50     watts 


ELECTRO-THERAPEUTICS.  133 

would  be  expended  in  the  substance  of 
the  wire  as  heat.  The  apparent  C.  E.  M. 
F.,  which  is  produced  in  a  circuit  by  its 
resistance  when  overcome  by  a  current, 
expends  activity  in  heat ;  whereas,  the 
C.  E.  M.  F.,  which  is  due  to  chemical 
decomposition,  or  to  magnetic  action,  ex- 
pends activity  chemically  or  magnetically. 
In  other  words,  work  done  in  a  circuit 
against  the  C.  E.  M.  F.  of  resistance,  is 
work  expended  thermally  ;  and,  therefore, 
is  practically  irrecoverable,  while  work 
done  in  a  circuit  against  the  C.  E.  M.  F. 
of  chemical  decompositions,  or  of  magnetic 
action,  is  capable  of  being  partly  or  almost 
entirely  utilized. 

Fig.  51,  represents  an  electric  calorim- 
eter;  i.  e.,  a  device  for  measuring  an 
electric  current  by  the  amount  of  heat 
produced  in  a  wire  which  is  immersed  in  a 


134  ELECTRICITY   IN 

known  volume  of  water.  In  the  form  of 
calorimeter  shown  in  the  figure,  the  cur- 
rent enters  and  passes  through  the  resist- 
ance coil  JVM,  surrounded  by  a  known 
quantity  of  water.     A  thermometer  T,  is 


Fig.   51. 

provided  for  measuring  the  increase  in 
temperature.  The  amount  of  electric 
energy,  which  must  be  expended  as  heat 
in  order  to  raise  the  temperature  of  one 
cubic  centimetre,  or  one  gramme,  of  water 
through  1°  C,  is   called   a  water-gramme- 


^iwR«^ 


degree-centigrade,  or  a  le^^^jalorie,  and 
is  approximately  equal  to  4.2joules.  Con- 
sequently, a  pound  of  water  being  453.6 
grammes,  when  raised  through  a  tempera- 
ture of  10°  C,  requires  an  expenditure  of 
energy  equal  to  4,536  water-gramme- 
degrees,  and  4,536  X  4.2  =  19,051  joules. 

In  the  case  of  a  current  of  25  milli  am- 
peres passing  through  the  human  body, 
under  a  pressure  of  20  volts,  and  repre- 
senting an  activity  of  0.5  joule-per-second, 
or  0.5  watt,  a  certain  amount  of  this  C.  E. 
M.  F.,  probably  2.5  volts,  would  be  due  to 
electrolytic  action,  and  the  remainder,  or 
17.5  volts,  due  to  the  resistance  of  the  body 
as  a  conductor.  The  activity  expended 
electrolytically  would,  therefore,  be  2.5  x 

25 

—^-r  =  0.0625    joule-per-second,    and    the 

remainder,  or  —^7^-  =  0.4375  ioule-per- 
1,000  J         r 


136  ELECTRICITY  IN 

second,  would  be  expended  in  warming  the 
conducting  materials  in  the  body.  The 
electrolytic  work  would  be  expended  at 
the  surface  of  the  metallic  electrodes,  while 
the  thermal  activity  would  be  expended 
wherever  the  resistance  was  overcome ; 
and,  moreover,  expended  in  proportion  to 
the  amount  of  resistance.  Assuming, 
however,  for  the  sake  of  illustration,  that 
the  amount  of  heat  so  developed  was 
equally  diffused  throughout  the  whole 
body,  and  that  the  capacity  of  the  body 
for  heat  was  that  of  100  lbs.  of  water, 
then  one  joule,  expended  in  the  body 
thermally,   would    raise    its    temperature 

4.2xl00X453.6=i9pl0  °f  a  de^ee  ^ 
tigrade.      0.4375     joule    would     raise     it 

'  of  a  degree  centigrade.     The  appli- 


ELECTRO-THERAPEUTICS.  137 

cation   of  the   current  for  30   minutes,  or 
1,800  seconds,  would  raise  its  temperature 

^-^r  of  a  degree  centigrade,  ap- 


190,510      242 
proximately. 

It  will  therefore  be  evident  that  the 
current  strengths  ordinarily  employed  in 
electro-therapeutics  cannot  directly  raise 
the  temperature  of  the  body  to  any  sensible 
degree,  although  physiological  activities 
called  into  action  thereby  may  do  so.  A 
current  of  several  amperes,  however,  passed 
through  the  body,  for  a  few  seconds,  will 
raise  the  temperature  appreciably. 


CHAPTER  VII. 

FRICTIONAL    AND    INFLUENCE   MACHINES. 

Our  earliest  notions  concerning  elec- 
tricity  were  obtained  from  the  electric 
effects  produced  by  the  friction  of  one  sub- 
stance against  another,  such,  for  example, 
as  a  piece  of  glass  against  a  silk  handker- 
chief. Under  these  circumstances,  both 
the  glass  that  is  rubbed  and  the  silk  with 
which  it  is  rubbed,  acquire  electric  ex- 
citement, as  manifested  by  their  ability  to 
attract  light  bodies,  such  as  shreds  of 
paper,  brought  near  them.  It  can  be 
shown  that  all  bodies  possess  the  ability 
of  acquiring  electric  excitement  by  fric- 
tion against  other  bodies,  and  that  when- 

138 


ELECTRO-THERAPEUTICS.  139 

ever  two  dissimilar  substances,  or  even  two 
dissimilar  surfaces  of  the  same  substance, 
are  brought  into  contact,  an  E.  M.  F. 
is  produced  at  the  contact  surfaces,  one 
substance  becoming  positive  and  the  other 
negative.  The  friction  of  one  substance 
against  another  is,  therefore,  another 
method  of  producing  an  E.  M.  F.,  and,  as 
in  the  case  of  the  other  sources  referred 
to,  this  E.  M.  F.,  if  provided  with  a  circuit, 
is  capable  of  setting  electricity  in  motion. 

The  E.  M.  F.'s  produced  by  friction, 
are  much  higher  than  those  produced  by 
either  voltaic  cells  or  dynamo-electric 
machines;  consequently,  they  are  capable 
of  causing  electricity  to  pass  through  a  cir- 
cuit even  when  separated  by  a  small  inter- 
val or  air-gap.  It  is  found  that  an  E.  M. 
F.  of,  approximately,  80,000  volts  is  re- 
quired  to  produce  a  discharge   or   spark 


140  ELECTRICITY  IN 

across  an  air-gap  one  inch  in  length,  be- 
tween two  slightly  convex  surfaces,  and 
that,  roughly,  80,000  volts  per  inch  of 
sparking  distance,  is  a  fairly  reliable  esti- 
mate of  pressure  for  distances,  lying 
between  1/100"  and  3".  Beyond  these 
limits,  the  rule  cannot  be  safely  applied, 
and,  in  fact,  for  very  large  sparking  dis- 
tances of  more  than  one  foot,  a  much 
smaller  E.  M.  F.  than  80,000  volts  per 
inch  appears  to  be  needed.  When  the 
opposed  electrodes  terminate  in  points, 
instead  of  in  blunt  surfaces,  a  relatively 
much  smaller  pressure  is  needed  to  effect 
discharge. 

Various  devices  have  been  employed  in 
order  to  produce  electricity  by  the  friction 
of  one  substance  against  another.  Such 
devices  are  called  frictional  electric  ma- 
chines,  and  consist    essentially  of  a  plate 


ELECTRO-THERAPEUTICS.  141 

or  cylinder,  generally  of  glass,  so  rotated 
as  to  be  rubbed  against  a  rubber  of 
chamois  skin,  covered  with  an  amalgam  of 
mercury  and  tin.  By  this  friction,  both 
the  rubber  and  the  glass  acquire  an  elec- 
tric potential.  A  comb  of  points  placed 
near  the  glass  and  connected  with  an 
insulated  conductor,  enables  the  conductor 
to  become  charged.  A  smaller  insulated 
conductor,  connected  electrically  with  the 
rubber,  enables  the  conductor  to  take  the 
opposite  or  negative  charge. 

A  well  known  form  of  frictional  elec- 
tric machine  is  shown  in  Fig.  52.  Here 
the  glass  plate  PP,  is  rotated  in  a  vertical 
plane  between  two  rubbers  J?,  i?,  of 
chamois  leather,  connected  electrically 
with  the  ground.  In  this  form  of 
machine,  two  insulated  conductors  C,  C, 
are  connected  to  separate  pairs  of  combs 


142 


ELECTRICITY   IN 


near   the   surface    of   the    revolving   glass 
plate  and  thus  receive  a  positive  charge. 


Fig.  52. 


When  an  electric  machine  is  properly 
operated  it  has  the  power  of  sending  a 
torrent  of  minute  sparks  through  a  con- 
siderable   air-space.     The    E.    M.   F.  pro- 


ELECTRO-THERAPEUTICS. 


143 


duced  by  this  type  of  source  is  of  the 
pulsatory  character,  and  is  shown  in  Fig. 
53,  here  represented  at  about  140,000  volts, 
or  140  kilo  volts. 


100- 
110- 

m- 


o    I 

d 

*    40H 


29- 


/FWflWFTHITK 


SECONDS. 


Fig.  53. 


When  a  pulsatory  E.  M.  F.  rises  to  such 
an  amount  as  will  permit  it  to  discharge 
through  an  air-gap,  it  suddenly  falls  on 
discharge  to  a  minimum  which  is  not 
always  the  same.  It  then  recovers  and 
again  discharges,  this  action  being  carried 


144  ELECTRICITY  IN 

on  in  a  pulsatory  manner  at  frequent  in- 
tervals. E.  M.  F.'s  of  this  character  are 
frequently  employed  in  electro-therapeu- 
tics under  the  name  of  Franklinic  E.  M. 
F^s,  after  Benjamin  Franklin.  A  fric- 
tional  electric  machine,  however,  is  too  un- 
certain in  its  action  to  be  employed  for 
this  purpose,  being  too  much  dependent 
on  the  conditions  of  the  weather,  since, 
during  damp  weather,  the  film  of  moisture 
which  settles  upon  the  surface  of  the  glass, 
and  on  the  supporting  pillars,  is  often 
sufficient  to  conduct  away  the  electric 
charges  and  prevent  their  formation.  For 
this  reason,  frictional  machines  have  been 
replaced  by  influence  machines  of  which 
there  are  a  number  of  different  designs. 

In  order  to  understand  the  operation  of 
an  influence  machine,  it  is  necessary  to 
first  investigate  what  occurs  in  the  space 


ELECTRO-THERAPEUTICS. 


145 


surrounding  an  electrically  charged  body. 
If  we  support  a  metallic  sphere  A,  upon 
a  table  in   a  room,  B  C  D  E,  Fig,   54, 


E  I 


Fig.  54. 


and  connect  this  sphere  with  an  E.  M. 
F.,  the  sphere  will  receive  a  charge.  If 
we  connect  the  sphere  by  a  wire  to  one 
terminal  of  a  voltaic  cell,  the  sphere  will 
become   charged,  thereby,  but  so   feebly, 


146  ELECTRICITY   IN 

that  delicate  apparatus  will  be  necessary 
in  order  to  reveal  the  presence  of  the 
charge.  If,  however,  we  connect  the 
sphere  to  one  terminal  of  a  battery  of 
10,000  cells,  each  having  an  E.  M.  F.  of 
one  volt,  an  appreciable  charge  will  be 
communicated  to  the  sphere,  which  will 
now  manifest  distinct  electrostatic  proper- 
ties. Finally,  if  we  connect  the  sphere  to 
one  terminal  of  an  electrostatic  machine, 
having  an  E.  M.  F.  of,  perhaps,  200,000 
volts,  the  charge  acquired  by  the  sphere 
will  be  comparatively  great.  That  is  to 
say,  it  will  receive  a  comparatively  large 
quantity  of  electricity,  which  will  be  a  cer- 
tain fraction  of  a  coulomb.  It  is  common 
to  regard  such  an  electric  charge  as  being 
situated  on  the  surface  of  the  body  A. 
Such,  however,  is  not  the  fact,  the  charge 
really  resides  in  the  air,  or  more  strictly 
in  the  air  and  the  ether  surrounding  the 


ELECTRO-THERAPEUTICS.  147 

body,  and  the  function  of  the  metallic 
cylinder  is  merely  to  provide  a  surface 
from  which  the  charge  can  enter  and 
influence  the  ether  around  it. 


If  we  assume,  for  the  sake  of  illustra- 
tion, that  the  charge  communicated  to  the 

sphere  is  -i  aaa  oonfo  °^  a  cou^om^  or  one 
micro-coulomb,  and  that  the  E.  M.  F.  at 
which  it  was  delivered  to  A,  was  100,000 
volts,  or,  in  other  words,  that  the  potential 
of  A,  is  100,000  volts,  then  the  work  de- 

livered  to  A,  is  100,000 x  lf)^m=  ^ 

joule =0.0738  foot-pound.  This  energy  is 
received  by  the  air  and  ether  surrounding 
the  sphere,  and  is  held  there  during  the 
maintenance  of  the  charge.  The  energy  is 
distributed  through  all  the  ether  in  the 
room,    although   not   equally    distributed. 


148  ELECTRICITY   IN 

A  certain  fraction  of  a.  joule  is  thereby 
charged  in  each  cubic  inch  of  space,  the 
greater  amount  being  in  the  immediate 
neighborhood  of  the  sphere,  and  lessening 
with  distance  from  the  same.  The  charge 
is  passed  into  the  ether  by  an  action  which 
is  called  electric  displacement.  Electric 
displacement  takes  place  along  defined 
lines  or  curves  through  the  ether  in  the 
room,  or  as  it  is  sometimes  stated,  electro- 
static flux  proceeds  from  the  charged  body 
through  the  ether  in  the  room  along  lines 
or  paths  called  lines  or  curves  of  electro- 
static flux.  The  shape  of  these  lines  de- 
pends on  the  shape  of  the  body,  and  on  the 
shape  of  the  enclosure  in  which  it  is 
placed,  or  of  the  bodies  which  may  be  in 
its  neighborhood.  Fig.  54,  gives  a  dia- 
grammatic view  of  some  of  these  displace- 
ment curves,  or  curves  of  electrostatic  flux. 
The   total    amount    of    electrostatic    flux 


ELECTRO-THERAPEUTICS. 


149 


is    proportional    to    the    charge    on     the 
body. 

Fig.  55,  gives  a  graphic  representation 
of  a  mechanical  model  showing  the  action 


Fig.  55. 


which  an  electrified  sphere  exerts  upon  the 
space  surrounding  it.  Let  j  h  I  ?n,  be  a 
diametral  section  of  a  spherical  elastic 
bag  of  rubber,  and  e  f  g  h,  the  section  of 
another  bag  of  rubber  surrounding  the  first 
concentrically.  Moreover,  suppose  these 
bags  to  have  the  space  between  them  en- 
tirely filled  with  water.     If 


now,  air  be 


150  ELECTRICITY   IN 

pumped  into  the  interior  of  the  inner  bag 
it  will  distend,  and,  in  distending,  will 
cause  the  outer  bag  to  also  distend,  al- 
though to  a  lesser  degree,  since  the  water 
between  them  is  practically  incompressible. 
The  inner  bag  corresponds  to  the  metallic 
sphere  of  Fig.  54,  and  the  outer  bag  cor- 
responds to  the  walls  of  the  room,  in 
which  the  sphere  A,  is  suspended.  The 
water  filling  the  space  between  the  bags 
corresponds  to  the  ether  filling  the  space 
between  the  sphere  and  the  walls  of  the 
room.  The  air  pressure  communicated  to 
the  interior  bag  by  pumping  air  into  it, 
corresponds  to  the  electric  pressure,  or  high 
potential  communicated  to  the  sphere. 
Under  the  influence  of  this  pressure,  the 
inner  bag  expands  or  receives  a  charge,  the 
expansion  causes  liquid  flux  or  streamings 
through  all  the  mass  of  liquid,  which  dis- 
tends the  outer  sphere.     This  corresponds 


On 

to  tlie  fact  that  the  cltaiige  communicated 
to  the  sphere  is  imparte^^O'^he,,  surround- 
ing ether  and  passes,  in  the  form  of  electro- 
static flux,  through  the  whole  ether  space 
until  intercepted  by  the  walls  of  the  room. 
The  total  charge  of  the  sphere  may  be  re- 
garded as  being  equal  to  the  total  dis- 
placement and  also  numerically  equal  to 
the  total  negative  charge  accumulated  on 
the  walls  of  the  room. 

In  order  to  show  the  effect  of  the  shape 
of  a  body  on  the  direction  of  the  flux 
paths,  a  few  examples  may  be  noted.  Fig. 
56,  shows  two  insulated  concentric  spheres 
connected  with  an  E.  M.  F.,  the  inner 
sphere  being  positive.  Here  the  flux 
issues  from  the  inner  sphere  in  radial 
lines,  and  terminates  on  the  surface  of  the 
outer  sphere.  The  total  amount  of  dis- 
placement    which     passes     through     any 


152  ELECTRICITY   IN 

spherical  envelope,  which  could  be  drawn 
between  A  and  B,  is  equal  to  the  charge 
which  resides  on  A  or  B.  In  other  words, 
the  system  behaves  as  though  a  certain 
amount  of  ether  had  been  liberated  at  the 
surface  of  A,  passed  through  the  surround- 


Fig.  56.— Electrostatic  Flux  Paths  Between  Con- 
centric Spherical  Conducting  Surfaces,  Insulated 
prom  Each  Other. 

ing  space  against  elastic  reaction,  along 
radial  lines,  until  finally,  a  certain  amount 
is  forced  into  the  shell  of  the  external 
sphere  at  B.  The  quantity  of  charge 
which  thus  enters  the  system  depends 
upon  two  things;  namely,  the  magnitude 


ELECTRO-THERAPEUTICS.  153 

of  the  E.  M.  F.  employed,  and,  secondly, 
the  shape  of  the  mass  of  ether  brought 
under  the  action  of  this  force.  If  we 
double  the  E.  M.  F.,  we  double  the 
charge,  and  if  we  halve  the  E.  M.  F.  we 
halve  the  charge.     Again,  if  we  employ  a 


Fig.  57. — Electrostatic  Flux  Paths  Between 
Insulated  Concentric  Spheres. 

thinner  stratum  of  ether  we  shall  reduce 
its  resiliency,  and  increase  the  charge  for  a 
given  E.  M.  F.  employed.  If,  for  example, 
the  inner  sphere  be  larger,  as  shown  in 
Fig.  57?  the  same  E.  M.  F.  will  now  act 
upon  a  thinner  layer  of  air  and  ether,  and 
will    produce   a   greater   displacement   or 


154 


ELECTRICITY   IN 


charge ;  in  other  words,  the  resiliency  of  the 
mass  of  ether,   enclosed  between  the  ter- 


-->-->—>--> 


Fig.  58. — Electrostatic  Flux  Paths,  Parallel 
Plane  Spheres. 

minals  connected  with  the  E.  M.  F.,  has 
been  reduced. 


Fig.    58,  represents  two    parallel  plane 
surfaces   connected    with    an    E.    M.    F, 


ELECTRO-THERAPEUTICS.  155 

Here  the  left-hand  plane  A,  is  considered 
as  positive ;  i.  e.,  the  electrostatic  flux  is 
assumed  to  emanate  from  A,  pass  through 
the  intervening  space,  and  terminate  at  the 
surface  of  B.  There  will  be  a  positive 
charge  on  A,  an  equal  negative  charge  on 
B,  and  the  same  charge  represented  in  dis- 
placement all  through  the  mass  of  ether  be- 
tween the  plates.  The  amount  of  charge 
which  will  enter  the  system,  will  depend 
upon  the  E.  M.  F.  brought  to  bear  upon 
the  plates  A  and  B,  the  thickness  of  the 
stratum  of  ether,  and  the  area  of  the  plates 
or  stratum.  If  the  area  of  the  opposed 
plates  be  increased,  the  elastic  resiliency  of 
the  mass  of  ether  between  the  plates  is  di- 
minished, and  a  proportionally  greater 
charge  enters  the  system.  Similarly,  if  the 
plates  be  approached,  so  that  the  stratum 
of  included  air  becomes  thinner,  its  resili- 
ency is  diminished,  and   the  E.  M.  F.  will 


156  ELECTRICITY  IN 

force  more  flux  through  the  system,  and  a 
corresponding  greater  charge.  In  either 
case,  therefore,  we  have  what  might  be  re- 
garded as  an  electrostatic  circuit. 

The  amount  of  flux  which  will  pass 
through  the  circuit ;  i.  e.,  the  amount  of 
charge  which  can  be  communicated  to  the 
surface  of  the  dielectric  involved,  will 
depend  upon  the  E.  M.  F.,  and  also 
upon  the  elastic  resistance  of  the  medium. 

E   M  F 

Electrostatic  flux=-pn — ; '  .  * — '- 

Electrostatic  resistance. 

The  greater  the  electrostatic  resistance,  the 
less  the  flux,  and  vice  versa.  This  corre- 
sponds completely  to  Ohm's  law  for  the 
voltaic  circuit,  except  that  the  electrostatic 
resistance  is  not  a  resistance  to  the  passage 
of  electric  current  but  is  the  resistance  to 
the  passage  of  electrostatic  current  or  flux. 
Moreover,    the   same   rules    apply   to   the 


ELECTRO-THERAPEUTICS.  157 

resistance  offered  by  a  wire  to  the  passage 
of  a  current,  and  the  resistance  offered  by 
a  dielectric  mass  to  the  passage  of  an 
electrostatic  flux.  The  longer  the  mass  the 
greater  the  resistance ;  the  greater  its  area 
of  cross-section,  the  smaller  its  resistance. 

The  displacement  lines,  or  lines  of 
electrostatic  flux,  which  may  be  drawn 
for  any  completely  specified  electrostatic 
system,  and  which  can  be  experimentally 
determined  in  most  cases,  represent  lines 
in  the  dielectric  medium  along  which 
stress  exists,  by  virtue  of  the  electrostatic 
flux.  This  stress,  which  is  developed  in 
the  ether,  is  dependent  upon  the  energy 
absorbed  by  the  ether  during  the  existence 
of  the  electric  charge.  Along  these 
curves,  in  fact,  there  is  exerted  a  continual 
tension,  or,  in  other  words,  the  displace- 
ment lines  are  always  tending  to  contract 


158  ELECTRICITY  IN 

and  shorten.  For  example,  the  two 
charged  plates  A  and  B,  shown  in  Fig.  58, 
being  connected  by  a  number  of  displace- 
ment lines,  tend  to  attract  each  other. 
The  real  tendency  is  for  the  shortening  of 
the  lines  of  stress,  or  flux  lines.  The  ordi- 
nary statement  that  positively  and  nega- 
tively' electrified  bodies  tend  to  attract 
each  other,  should  more  accurately  be : 
positively  and  negatively  electrified  bodies, 
being  connected  by  lines  of  electrostatic 
flux,  tend  to  come  together  by  reason  of 
the  contraction  of  the  flux  lines. 

If,  as  in  Fig.  59,  a  small  spherical  con- 
ductor C,  be  introduced  into  the  electrostatic 
flux,  the  effect  is  twofold  ;  first,  the  electric 
medium  is  thinned  locally  by  the  presence 
of  the  conductor,  so  that  its  resiliency  is 
locally  diminished,  and  a  more  powerful 
flux  will  pass  through  the  system  in  its 


ELECTRO-THERAPEUTICS.  159 

neighborhood  than  elsewhere.  Second, 
the  flux  will  be  intercepted  by  the  con- 
ductor, which  will  form  the  termination 
of  the  flux  on  the  entering  side  and  a  new 
starting  point  on  the  leaving  side.  A 
negative  charge  will,  therefore,  appear  on 
the  surface  where  the  flux  terminates,  and 
a  positive  charge  on  the  surface  where  the 
flux  reissues.  This  appearance  of  posi- 
tive and  negative  charges,  on  opposite 
sides  of  an  insulated  body  supported  in 
an  electrostatic  flux,  is  commonly  called 
electrostatic  induction.  It  is  merely  a 
consequence  of  the  fact  that  the  body 
relieves  from  electrostatic  stress  the  ether 
which  it  displaces,  and  that,  in  conse- 
quence of  this  relief,  charges  appear  at  the 
surfaces  where  the  flux  enters  and  leaves. 

If  in  Fig.  59,  the  small  conductor  C,  is 
charged  by  having  been  connected  with  a 


160 


ELECTRICITY   IN 


suitable  E.  M.  F.  it   will  do  more  than 
merely  relieve  ether  of  its  duties;   for,  it 


"A 
+ 


Fig.  59.— Diagrammatic  Representation  of  Electro- 
static Flux  Paths. 

will  add  flux  of  its  own  to  the  flux  in  which 
it  is  introduced.  For  example  in  Fig.  60, 
two  spheres  +  and  — ,  are   shown,  at  A, 


ELECTRO-THERAPEUTICS.  161 

which  have  been  connected  with  the  posi- 
tive and  negative  terminals  of  a  high 
E.  M.  F.  The  electrostatic  flux  passing 
between  them,  through  the  surrounding 
ether,  is  partly  represented  diagrammati- 
cally  by  the  dotted  lines.  These  two 
spheres  evidently  behave  as  though  they 
attracted  each  other,  owing  to  the  contract- 
ing forces  of  all  the  flux  paths  between 
them.  If  we  suppose  them  fixed  upon 
suitable  insulating  pillars,  so  that  they 
cannot  approach  each  other,  and  that  a 
smaller  conducting  sphere  is  introduced 
between  them,  as  shown,  this  smaller 
sphere  will  acquire  a  positive  charge  from 
the  positive  sphere,  and  will  thus  become 
the  recipient  of  a  number  of  flux  paths 
which  emerge  from  it,  which  tend  to  pull  it 
across  toward  the  negative  sphere.  Under 
the  influence  of  these  attractive  forces, 
the  smaller  sphere,  if  it  be  free  to  move, 


162  ELECTRICITY   IN 

will  move  to  the  right.  When  it  is  in 
the  position  shown  on  the  right,  midway 
between  the  two  spheres,  it  will  be  seen 
that  many  flux  paths  connect  it  with  the 
negative  sphere,  while  no  flux  paths  con- 
nect it  with  the  positive  sphere.     As  soon 


/   - — -« 


A 

Fig.  60.— Effect  of  Charged  Sphere. 

as  it  reaches  the  negative  sphere  it  will 
deliver  up  its  charge,  and  reduce  the  po- 
tential of  the  negative  sphere  unless  the 
latter  be  connected  with  an  electric  source. 
It  will  then  acquire  a  negative  charge  and 
new  flux  paths  will  enter  its  surface  from 
the  positive  sphere.  Owing  to  the  attrac- 
tion of  these  flux  lines  it  will  again  be 
drawn  to  the  left  and  thus  a  continual  to- 


PROPERTY  of' 


ELECTRO-THER 


and-fro  motion  will  be  set 
difference  of  potential  or  E.  M. 
between  the  two  large  spheres. 


Fig.   61,  represents  at  A,  the  effect  of 
inserting,  between  the  two  large  spheres, 


^W^j 


A  B 

Fig.  61.— Effect  of  Uncharged  Sphere. 

a  small  sphere  in  an  uncharged  condition. 
It  will  be  observed  that  the  effect  is  to 
intercept  a  larger  number  of  flux  paths, 
and  thus  to  relieve  from  duty  the  ether 
contained  within  the  space  occupied  by 
the  small  sphere.  In  this  case  the  attrac- 
tions, on  each  side  of  the  small  sphere,  are 
balanced.  If,  however,  the  small  sphere 
be  placed  nearer  one  side  than  the  other, 


164  ELECTRICITY   IN 

as  shown  at  B,  the  stratum  of  ether 
between  it  and  the  positive  sphere  will  be 
thinner  than  the  stratum  on  the  right ;  and, 
consequently,  a  greater  electrostatic  flux 
will  pass  through  the  space  on  the  left, 
thereby  entailing  the  introduction  of  a 
greater  number  of  flux  lines,  and  a  greater 
electrostatic  force  urging  the  sphere  to  the 
left. 

It  will  be  seen,  from  a  consideration  of 
the  preceding  phenomena,  that  the  follow- 
ing may  be  generalized  as  the  laws  of 
electrostatic  charges,  attractions  and  repul- 
sions; namely, 

(1)  That  every  electrified  body  forms  a 
locus  or  place  where  electrostatic  flux 
enters  or  leaves  a  dielectric  medium ;  and, 
conversely,  that  all  conducting  surfaces, 
where  lines  of  electrostatic  flux  terminate, 
are  said   to  be  charged  surfaces,  the  flux 


ELECTRO-THERAPEUTICS.  165 

being  assumed  to  leave  at  positively 
charged  surfaces  aud  to  enter  at  negatively 
charged  surfaces. 

(2)  Lines  of  electrostatic  flux  are  the 
directions  along  which  electrostatic  stress 
exists  in  the  ether,  and  accompany  the 
temporary  absorption  of  energy  into  the 
ether. 

(3)  Dissimilarly  charged  bodies  attract 
one  another,  because  lines  of  electrostatic 
flux  tend  to  contract. 

(4)  That  similarly  charged  bodies  ap- 
pear to  repel,  owing  to  the  fact  that 
no  flux  paths  connect  them,  but  that  flux 
paths  connect  each  of  them  with  neigh- 
boring objects,  so  that  they  are  drawn 
to  the  neighboring  bodies  and  are  not 
repelled  from  each  other. 

(5)  Electrostatic  induction  accompanies 
the  introduction  of  a  conductor  into  the 
electrostatic    flux,    whereby     charges    are 


166  ELECTRICITY   IN 

caused    to   form    upon    orrposite    sides   of 
the  interposed  conductor. 

The  principle  of  electrostatic  induction 
is    employed    in    influence    machines.     A 


Fig.  62.— Electrophorus. 

simple  form  of  influence  machine  is  called 
the  elect rophoros,  and  is  illustrated  in 
Fig.  62.  A  disc  £,  of  vulcanite,  resin, 
or  other  suitable  material,  is  vigorously 
rubbed,  say  with  a  cat  skin,  and  thereby 
becomes  negatively  charged,  under  the 
influence  of   the   powerful   E.    M.  F.    set 


ELECTKO-THERAPEUTICS.  167 

up.  If  such  an  electrified  disc  be  laid  on 
a  table,  as  shown  in  Fig.  63,  so  that  its 
electrified  surface  is  uppermost,  the  flux 
paths  may  be  represented  diagrammati- 
cally  b}^  the  arrows. 


Fig.  63. — Representing  Action  and  Operation 
of  electrophoru8. 


If  now,  an  insulated  metallic  disc  A, 
Figs.  62,  and  64,  furnished  with  round 
edges,  be  rested  on  the  disc,  there  will  be 
no  great  change  produced  in  the  electro- 
static system.  There  will  only  be  a  slight 
reduction  in  the  dielectric  resistance  of  the 
air,  owing  to  the  interposition  of  the  con- 


168  ELECTRICITY   IN 

ducting  disc  across  the  electrostatic  flux 
paths.  When,  however,  the  disc  is 
touched  with  a  finger,  as  shown  in  Fig. 
62,  or  connected  with  the  ground,  as 
shown  in  Fig.  65,  all  the  flux  paths  are 
shortened,  until   they  exist  only  between 


'   „-  — v, 

1 ;  r<l 


t  /„-.. 


1/ *    i     * 


Fig.  64. — Representing  Action  and  Operation  of 
Electrophorus. 


the  excited  disc  and  the  grounded  metallic 
plate.  In  other  words,  the  length  of  the 
electrostatic  circuit  has  been  reduced  to 
that  of  a  thin  film  of  air,  and,  con- 
sequently, the  electrostatic  flux,  set  up 
across  this  film,  will  be  comparatively 
powerful,  and  a  powerful  charge  will  be 


ELECTRO-THERAPEUTICS.  169 

communicated  to  the  plate  at  the  point 
where  the  flux  emerges  from  it.  If  now, 
the  ground  connection  to  the  plate  be 
removed,  and  the  plate  lifted,  as  shown 
in  Figs.  66  and  67,  the  electrostatic 
circuit   will   again   be   lengthened,  and  a 


f 


Fig.  65.— Electrophorus. 

charge  will  be  left  in  the  plate  as  well 
as  on  the  disc,  the  disc  being  negative 
and  the  plate  positive.  The  metal  plate 
can  be  discharged  and  recharged  many 
times  in  succession. 

An  influence  machine  is,  in  reality, 
a  form  of  a  revolving  electrophorous. 
A   common    form   of    influence   machine, 


170  ELECTRICITY   IN 

called  a  ToepUr-Holte  Machine  is  shown 
in  Y\v.  68.  It  will  be  understood  that 
since  this  apparatus  operates  by  electro- 
static induction,  no  friction  is  needed.  An 
initial  charge  is,  however,  required.  The 
apparatus    consists    essentially    of    three 


/vmtHmtttttttttr^ 

t  HttttttHtttttntttt// 

1         ■    —L_ 

Fig.  66.— Electrophorus. 

vertical  glass  plates  of  which  the  central 
is  the  largest  in  diameter,  and  is  fixed, 
while  the  two  outside  plates  are  mounted 
on  a  common  shaft,  and  are  capable  of 
being  revolved  in  the  same  direction  by 
the  aid  of  the  handle  h.  The  central  plate 
carries  two  metallic  and  papered  surfaces 


# 


^^^SEtfj^ 


^L^CTUo-TmmAVT^dff^p  r  P  7^1 


% 


CF 


Q> 


e/ 


A  B  Ctm&A'  B  C^%)f  which  A  B  'c, 
is  positively  electrified  ik^^ev  outset  aqd 
J.'  ^'   0*,  is   negatively  elec^Sisi^Bia^- 
outside  plates  carry  only  metallic  buttons 
on   their   external    surfaces,    each    button 


Fig.  67. — Electrophorus. 

consisting  of  a  disc  of  tinfoil,  with  a  small 
brass  cap  in  the  centre.  Six  of  these  tin- 
foil discs  are  represented  as  being  carried 
on  each  outside  plate. 

An   electrostatic  circuit  will  be  set  up 
from  the  positively  electrified  to  the  nega- 


172 


ELECTRICITY   IN 


tively  electrified  surface  as  shown  at  A,  in 
Fig.  69.  The  presence,  however,  of  the 
metallic  rod  i?i?,  which  is   supported  in 


Fig.  68. 


-Triple-Plate  Toepler-Holtz  Electrical 
Machine. 


such  a  manner  that  the  combs  at  its  ex- 
tremities come  in  contact  with  the  insu- 
lated discs  on  the  outside  plates  as  they 
revolve,  limits  the  electrostatic  circuit  al- 
most  entirely  to   the   space   between  the 


ELECTKO-THERAPEUTICS. 


173 


central  and  outside  plates,  as  shown  at  B, 
Fig.  69,  so  that  the  flux  paths  become 
more  numerous  and  terminate  on  the  inner 


VT4  vv^ 


-^tt^ 


->*•• — *-^» 


B  + 


a 


bT- 


za 


Fig.  69.— Electrostatic  Circuits  of  Influence 
Machine. 


surfaces  of  the  insulated  discs  as  they  pass 
by.  Under  these  circumstances,  a  nega- 
tive charge  will  form  on  the  disc  a?,  and  a 
positive  charge  on  the  disc  y.     As  soon  as 


174  ELECTRICITY  IN 

the  discs  have  been  carried  from  beneath 
the  combs  on  the  rod  HH,  they  retain 
these  charges  until  they  reach  the  opposite 
side  of  the  frame,  when  the  disc  a?,  comes 
in  contact  with  the  brush  b',  thereby  com- 
municating its  charge  to  the  already  nega- 
tively electrified  surface  A'  B'  C>  on  the 
central  plate,  and,  passing  with  the  re- 
mainder of  its  charge,  delivers  this  re- 
mainder to  the  comb  of  points  attached  to 
the  handle  and  main  conductor  IF.  Simi- 
larly, the  disc  y,  which  retains  its  positive 
charge  after  quitting  the  comb  on  the 
lower  extremity  of  the  rod  MR,  is  carried 
to  the  brush  b,  and  communicates  its  charge 
to  the  already  positively  electrified  surface 
A  B  C,  the  remainder  of  its  charge  being 
collected  by  the  comb  on  the  handle  H. 
Consequently,  during  rotation,  the  half  of 
the  rotating  plates  on  one  side  of  the  rod 
Hit,  is  positively,  and  the  other  half,  nega- 


ELECTRO-THERAPEUTICS.  175 

tively  electrified.  The  charges  on  the 
electrified  surfaces  A  B  Cand  A!  B  (7/  auto- 
matically increase,  until  a  balance  is  main- 
tained between  the  further  accession  of 
charge,  and  the  leakage  which  takes  place 
between  them.  This  leakage  limits,  there- 
fore, the  maximum  E.  M.  F.  obtainable  by 
the  machine.  When  the  discharging  rods 
H,  Hj  are  brought  close  together,  the  pres- 
sure obtained  is  lower,  owing  to  the  fact 
that  a  smaller  E.  M.  F.  is  required  to  pro- 
duce a  spark  discharge  across  the  air-gap, 
and  a  more  rapid  stream  of  discharges  over 
this  air-gap  and  a  lower  pressure  may,  con- 
sequently, be  expected.  On  increasing  the 
distance  between  the  discharging  rods,  the 
pressure  increases,  but  the  frequency  of 
discharge  usually  diminishes. 

A  form  of  apparatus  known  as  a  con- 
denser  consists  essentially  of  an   electro- 


176  ELECTEICITY   IN 

static  circuit  of  low  resistance,  that  is  to 
say,  of  an  electrostatic  circuit  of  short 
length  and  large  cross-sectional  area.  A 
condenser,  therefore,  offers  a  comparatively 
small  elastic  resistance  to  displacement  of 
flux,  and,  under  a  given  E.  M.  F.,  will  re- 
ceive a  correspondingly  large  charge. 

Fig.  70,  shows  a  Ley  den  jar,  which  is 
the  usual  form  of  condenser  employed  with 
high  E.  M.  F.'s.  Here  the  active  sur- 
faces are  formed  of  inner  and  outer  coat- 
ings of  tin-foil,  and  the  dielectric  consists 
of  the  glass  walls  of  the  jar.  The  length 
of  such  an  electrostatic  circuit;  i.  e.y  the 
thickness  of  the  glass,  may  be  about  l/8th 
of  an  inch,  and  the  cross-sectional  area  of 
the  electrostatic  circuit ;  i.  e.,  the  area  of 
the  tin-foil  surface,  about  a  square  foot. 
Moreover,  glass  offers  less  electrostatic 
resistance  than  air,  and,  therefore,  the  glass 


ELECTRO-THERAPEUTICS. 


177 


Leyden  jar  makes  a  better  condenser  than 
an  imaginary  air  jar  of  the  same  dimen- 
sions.    The  relative  value  of  the  glass  de- 


II 


Fig.  70.— Leydes  Jar. 


pends  upon  its  quality,  but  it  may  readily 
offer  five  times  less  electrostatic  resistance 
than  air ;  consequently,  the*  capacity  of  a 


178  ELECTRICITY   IN 

Leyden  jar  condenser  may  be  five  times 
greater  than  that  of  a  similar  air  con- 
denser. Two  small  Leyden  jars  are  shown 
in  Fig.  68,  having  their  inner  coatings  con- 
nected with  the  main  terminals  i7  and  H\ 
and  their  outer  coatings  connected  by  a 
metallic  strip  not  shown  in  the  figure. 
The  effect  of  these  jars  is  to  diminish  the 
electrostatic  resistance  between  the  termi- 
nals, and,  therefore,  enables  a  given  E.  M. 
F.  to  accumulate  -a  greater  electrostatic 
flux  or  charge  between  the  terminals. 

The  electric  energy  obtained  from  the 
discharge  of  an  influence  machine  through 
an  external  circuit  is  supplied,  mechanic- 
ally, in  the  effort  necessary  to  revolve  the 
machine  against  electrostatic  forces.  One 
electrostatic  machine  acting  as  a  gener- 
ator, may  readily  be  made  to  cause  an- 
other electrostatic  machine    to   run   back- 


Fig.  71.— Holtz  Influence  Machine. 


wards,  as  a  motor.  The  hand  has,  there- 
fore, to  be  applied  with  greater  force  to 
drive  the  influence  machine,  owing  to  the 


180 


ELECTRICITY   IN 


fact  that  it  is  operating  so  as   to  furnish 
current  to  the  circuit  connected  to  it. 


Fig.  72. — Bonette  Electrostatic  Influence  Machine. 

A  number  of  forms  of  influence  ma- 
chines are  in  existence.  The  principal 
difficulty  in  operating  such  machines  is  to 
maintain  their  insulation  during  all  condi- 


ELECTRO-THERAPEUTICS.  181 


Fig.  73. — Wimshurst  Electrical  Machine. 


182  ELECTRICITY   IN 

tions  of  weather,  so  that  their  charge  shall 
not  be  lost.  For  this  purpose  they  are 
often  enclosed  in  glass  chambers  in 
which  the  air  is  kept  dry  by  some  hygro- 
scopic substance,  such  as  calcium  chloride. 
Such  a  form  of  machine  driven  ■  by  a  small 
electric  motor  is  shown  at  Fig.  71. 

In  some  forms  of  influence  machines,  in 
order  to  ensure  the  presence  of  a  small 
charge,  a  small  frictional  attachment  is 
supplied,  so  that  the  proper  charge  shall 
be  ensured  by  the  friction  set  up. 

Glass  plates  are  not  invariably  used  in 
these  machines.  Sometimes  plates  of 
hard  rubber  are  employed  as  shown  in 
Fig.  72. 

Another  convenient  form  of  influence 
machine  is  shown  in  Fig.    73    called  the 


ELECTRO-THERAPEUTICS.  183 

Wimshurst  machine.  In  this  case  two 
glass  plates,  supporting  a  number  of  small, 
separate  tin-foil  conductors,  are  rapidly 
driven  in  opposite  directions.  The  action 
of  the  machine  differs  only  in  detail  from 
that  already  described. 

In  conclusion,  we  may  observe  that  all 
electrostatic  influence  machines  depend  for 
their  operation  upon  the  principle  of  the 
electrophorus.  The  electrostatic  circuit  in 
such  machines  is  periodically  lengthened 
and  shortened,  and  the  charges  so  induced 
are  separated  and  accumulated. 


CHAPTER  VIII. 

MAGNETISM. 

Magnetism  is  the  science  which  treats 
of  the  properties  and  laws  of  magnets 
whether  artificial  or  natural.  Although 
the  nature  of  magnetism  is  not  known,  yet 
a  certain  relationship  unquestionably  ex- 
ists between  magnetism  and  electricity,  so 
that  a  knowledge  of  the  nature  of  one 
must  inevitably  determine  a  knowledge 
of  the  nature  of  the  other.  Both  are 
believed  to  be  active  conditions  of  the 
universal  ether  and  are  so  related  that  the 
following  laws  appear  to  hold  generally ; 
viz., 

(1)  A  motion  of  electricity  invariably 
produces  magnetism. 

184 


ELECTRO  THERAPEUTICS. 


185 


(2)  A  motion  of  magnetism  invariably 
produces  an  E.  M.  F. 

The  nature  of  the  action   which  exists 
between  electricity    and    magnetism   may 


Fig.  74.— Hydraulic  Analogy  op  Relation  Between 
Electricity  and  Magnetism. 

be  illustrated  by  the  following  hydraulic 
experiment.  Suppose  that  a  large  cy- 
lindrical tank,  represented  in  Fig.  74,  be 
completely  filled  with  water,  and  that  a 
plunger  JP,  is  provided  with  a  rod  t,  pass- 


186  ELECTRICITY   IN 

ing  through  water-tight  packing  in  the 
centre  of  one  circular  end.  It  is  evident 
that  if  the  rod  t,  be  moved  forward,  the 
plunger  jP,  will  advance  into  the  tank.  In 
so  doing  it  will  displace  the  water  in  front 
of  it,  which  will  flow  round  to  the  back  of 
the  plunger  in  vortical  paths,  formed  sym- 
metrically around  the  face  of  the  plunger. 
These  vortical  paths,  passing  from  the 
front  to  the  back*  of  the  plunger,  are  illus-. 
trated  diagrammatically  at  A,  Fig.  74. 
The  vortical  movement  of  the  water  will 
clearly  be  most  marked  in  the  immediate 
neighborhood  of  the  eds*es  of  the  advanc- 
ing  plunger,  gradually  decreasing  from 
the  edges  to  the  sides  of  the  tank.  Imagi- 
nary  lines  in  the  mass  of  the  water,  drawn 
so  as  to  represent  the  intensity  of  the 
vortical  movement,  form  circles,  concentric 
to  the  axis  of  the  plunger,  and  at  right 
angles  to  the  direction  of   its  motion,  as 


ELECTRO-THERAPEUTICS.  187 

represented  at  B1  Fig.  74.  Circles  are 
marked  with  long  arrows  near  the  edge  of 
the  plunger  where  the  vortical  motion  is 
most  intense,  and  with  shorter  and  shorter 
arrows  at  greater  distances  from  it.  If 
now,  we  remove  the  plunger  from  the  tank, 
and  artificially  cause  a  system  of  electric 
currents  to  be  produced  in  the  mass  of 
quiescent  water,  such  as  is  represented  at 
B,  in  Fig.  74,  then  accompanying  this 
system  of  electric  currents,  would  be  pro- 
duced a  magnetic  distribution  throughout 
the  water,  such  as  is  represented  by  the 
stream  lines  at  A,  at  right  angles  to  the 
direction  of  the  electric  current.  In  other 
words,  the  relation  of  magnetic  distribu- 
tion, to  electric  current  distribution,  in  any 
space,  is  identical  with  the  relation  be- 
tween the  stream  lines  of  motion  in  a 
liquid,  and  the  vortical  distribution  of 
motion  accompanying  the    same. 


188  ELECTRICITY   IN 

It  follows  from  the  preceding  that  if 
the  all-pervading  ether  were  a  non-com- 
pressible fluid,  like  water,  and  if  electric 
currents  consisted  of  vortices  or  whirls  in 
this  fluid,  that  magnetism  would  consist  of 
a  streaming  motion  in  the  ether.  The  prop- 
erties of  the  ether  are  not  yet  thoroughly- 
known,  and  it  is  by  no  means  certain  that 
electric  currents  are  vortices  therein.  All 
that  can  be  safely  asserted  is  the  existence 
of  a  relationship  between  electric  activity 
and  magnetic  activity  in  the  universal 
ether,  of  the  general  nature-  we  have  here 
pointed  out ;  so  that,  if  we  should  at  any 
time,  discover  the  nature  of  either  electric- 
ity or  magnetism,  the  nature  of  the  other 
would  be  immediately  deduced. 

Magnetism  may  be  produced  in  two 
ways ;  viz., 

(1)   By  permanent  magnets   of   iron  or 


ELECTRO-THERAPEUTICS.  189 

steel ;  or,  in  a  lesser  degree,  by  other  mag- 
netic metals  such  as  nickel  or  cobalt ;  and, 
(2)  By  electric  currents. 

Magnetism  appears  to  be  an  inherent 
property  of  the  molecules,  or  ultimate 
particles,  of  iron  or  steel.  In  other 
words,  if  we  could  isolate  and  perceive  a 
single  molecule  of  iron,  it  is  believed  that 
we  should  find  that  it  naturally  and  per- 
manently possessed  magnetism,  as  a  prop- 
erty inherent  in  it.  If  the  ultimate  par- 
ticles of  iron  are  essentially  magnetic, 
the  question  naturally  arises,  why  all  iron 
does  not  manifest  magnetic  properties? 
The  reason  is  believed  to  be  found  in  the 
fact  that  in  iron,  which  is  apparently  un- 
magnetized,  the  molecules  lack  a  definite 
arrangement  of  direction,  and,  pointing 
irregularly,  mask  or  neutralize  each  other's 
magnetic  influence.     Such    an    undirected 


190  ELECTRICITY  IN 

system  would,  therefore,  necessarily  pos- 
sess no  appreciable  external  magnetism. 
When  a  bar  of  iron  is  magnetized,  it  is 
subjected  to  a  process  whereby  its  molec- 
ular magnets  are  aligned,  or  similarly 
directed,  and,  acting  in  concert,  are 
thereby  enabled  to  manifest  external  mag- 
netic effects. 

The  region  surrounding  a  magnet  is 
filled  with  what  is  called  magnetic  flux  or 
magnetism,  which  is  most  powerful  in  the 
immediate  neighborhood  of  the  poles.  If 
we  assume,  as  a  working  hypothesis,  that 
magnetism  consists  of  a  streaming  motion 
of  the  ether,  in  accordance  with  the 
hydraulic  analogue  of  Fig.  74,  then  we 
may  regard  a  magnet  as  a  device  for  pro- 
ducing such  a  streaming  motion  in  the 
ether.  The  magnetic  flux ;  i.  <?.,  the  stream- 
ing ether,  would  issue  from  one  pole  of  the 


ELECTRO-THERAPEUTICS.  191 

magnet,  and,  after  having  passed  in. ex- 
panding curved  paths  through  the  space 
surrounding  the  magnet,  would  re-enter  it 
at  the  other  pole.  The  pole  from  which 
the  magnetic  flux  is  conventionally  as- 
sumed to  issue  is  called  the  north  pole  / 
i.  e.,  the  pole  of  the  magnet,  which,  if 
the  magnet  were  freely  suspended,  would 
tend  to  point  toward  the  geographical 
north.;  and  the  pole  at  which  the  flux 
enters  the  magnet,  after  having  passed 
through  the  region  outside  it,  is  called  the 
south  pole.  The  flux,  after  entering  the 
magnet,  passes  through  the  body  of  the 
magnet  to  the  north  pole,  where  it  again 
emerges.  Magnetic  flux,  therefore,  com- 
pletes a  closed  path  or  circuit  called  a 
magnetic  circuit,  and  the  convention  em- 
ployed as  to  its  direction  in  this  circuit,  is 
similar  to  the  convention  employed  as  to 
the  direction  of   electrostatic   flux   in   an 


192  ELECTRICITY   IN 

electrostatic  circuit,  or  to  the  direction  of 
a  current  in  a  voltaic  circuit. 

When  an  electric  current  passes  through 
a    conductor,    the   conductor  temporarily 


Urn*.  i;m>: 


0 
/ 
^ 


Fig.  75.— Magnetic  Flux  Paths  Surrounding  a 
Straight  Active  Conductor. 

acquires  magnetic  properties,  magnetic 
flux  encircling  the  conductor  in  concentric 
paths.  The  direction  of  magnetic  flux 
around  an  active  conductor,  depends  on 
the  direction  of  the  current  in  the  con- 
ductor. This  is  shown  in  Fig.  75,  where, 
at  J5,  the  current  is  supposed  to  be  passing 


ELECTRO-THERAPEUTICS.  193 

through  the  wire,  in  a  direction  from  the 
observer.  Here  the  circles  surrounding 
the  wire  show  that  the  magnetic  flux  is 
passing  in  concentric  circles  in  the  direction 
of  motion  of  the  hands  of  a  clock ;  while  at 
A1  where  the  current  passes  through  the 
wire  in  a  direction  towards  the  observer, 
the  direction  of  the  magnetic  flux  around 
the  wire  is  opposite  to  the  direction  of 
motion  of  the  hands  of  a  clock,  or  counter- 
clockwise. A  suspended  magnetic  needle, 
introduced  into  the  neighborhood  of  the 
active  wire ;  i.  e.,  into  the  influence  of  its 
circular  magnetic  flux,  is  deflected  thereby, 
and  tends  to  set  itself  parallel  to  the  mag- 
netic flux,  or  at  right  angles  to  the  direc- 
tion of  the  current,  its  north  pole  pointing 
in  the  direction  of  motion  of  the  flux. 

The  power  possessed  by  an  active  con- 
ductor of  deflecting  a  magnetic  needle  is 


194  ELECTRICITY   IN 

utilized  in  a  number  of  ammeters,  in  which 
a  magnetic  needle  is  deflected  by  the  pas- 
sage of  a  current  through  a  number  of 
turns  of  wire  placed  in  its  vicinity.  When 
a  wire  carrying  an  electric  current  is  bent 
into  a  turn  or  loop,  all  the  magnetic  flux 
linked  with  the  wire  enters  this  loop  at 
one  face  and  leaves  it  at  the  other  face. 
Consequently,  that  face  of  the  loop  from 
which  the  flux  emerges  must  correspond 
to  the  north  magnetic  pole,  and  that  at 
which  it  enters,  to  the  south  magnetic  pole, 
of  an  ordinary  bar  magnet.  This  is  illus- 
trated in  Fig.  76,  both  in  the  case  of  a 
permanent  steel  magnet,  and  of  an  active 
coil.  In  the  case  of  a  magnet,  the  flux  is 
represented  as  coming  out  of  the  north 
pole,  as  indicated  by  the  arrows,  traversing 
the  region  or  space  outside  of  the  magnet, 
re-entering  the  magnet  at  its  south  pole, 
and  continuing  through  the  body  of  the 


*s* 


^U?  &  DIESfL  f%/ 


magnet  to  its  north  pole,^^s  completing 
the   magnetic   circuit.      Simil§rfe?4nr  ^e 
case  of  an  active  loop,  as  showi 
current  circulates  around  this  loop  clock- 
wise, as  viewed  by  an  observer  at  A,  then 


C'R   r 


S-^-lN 


-Z"  B 


Fig.  76. — Diagram  of  Flux  Produced  by  Permanent 
Magnet  and  by  Coil  of  Active  Conductor. 


the  flux  will  enter  at  A,  and  emerge  at  j&, 
so  that  the  face  B,  becomes  a  north  pole, 
and  A,  a  south  pole,  corresponding  to  the 
permanent  magnet.  In  the  case  of  an 
active  loop,  the  flux  paths  form  closed 
magnetic  circuits  as  in  the  case  of  the  mag- 


196  ELECTRICITY   IN 

net,  although  these  are  not  shown  in  the 
figure.  When  the  current  in  the  conduct- 
ing  loop  ceases,  the  magnetic  flux  linked 
with  the  loop  entirely  disappears. 

Magnetic  circuits  are  of  three  kinds; 
namely, 

(1)  Those  in  which  all  parts  of  the  path 
of  the  flux  are  completed  through  air,  or 
other  non-magnetic  material,  such  as  wood, 
copper,  glass,  etc.  Such  a  circuit  is  called 
a  non-ferric  magnetic'  circuit. 

(2)  Those  in  which  all  portions  of  the 
path  of  the  flux  are  completed  through 
iron  or  steel.  Such  a  circuit  is  called  a 
ferric  circuit. 

(3)  Those  in  which  parts  of  the  circuital 
path  of  the  flux  are  completed  through 
iron  or  steel,  and  parts  through  air  or  other 
non-conducting  material.  This  is  called 
an  aero-ferric  circuit 


ELECTRO-THERAPEUTICS. 


197 


Non-magnetic  circuits  are  formed  by 
active  conductors,  such  as  wires,  loops  or 
coils  carrying  electric  currents  in  the  ab- 


Fig.  77.— Ferric  Magnetic  Circuit. 


sence  of  iron.  An  example  of  such  a  cir- 
cuit is  represented  by  the  active  coil 
shown  in  Fig.  77.  Ferric  magnetic  cir- 
cuits are  less  frequently  met  with,  from 


198  ELECTRICITY   IN 

the  fact  that  the  object  of  sending  a  mag- 
netic flux  through  a  circuit  is  to  employ 
such  flux  in  the  operation  of  some  mechan- 
ism, generally  placed  in  a  gap  in  the  circuit 
itself.  There  is,  however,  a  compara- 
tively large  class  of  apparatus  called  alter- 
nating-current transformers,  which  will  be 
briefly  explained  later,  and  which  almost 
always  employ  ferric  circuits. 

An  iron  ring,  or  core,  wrapped  with  a 
coil  of  wire  connected  to  the  terminals  of  a 
battery,  is  an  example  of  a  ferric  magnetic 
circuit.  Such  a  magnetic  circuit  is  shown 
in  Fig.  77.  Here  all  the  flux  due  to  the 
active  conductors  is  entirely  confined  to 
the  iron  ring.  A  practical  form  of  a  ferric 
circuit  is  represented  in  Fig.  78,  which 
represents  an  alternating-current  trans- 
former. The  coil  of  active  conductor  is 
shown  at  AA,  linked  with  a  laminated  or 


ELECTRO-THERAPEUTICS.  199 

sheet  iron  core  BB.     Aero-ferric  magnetic 
circuits  are  commonly  observed  in  the  case 

1  


Fig.  78.— Alternating-Current  Transformer, 
Ferric  Magnetic  Circuit. 

of  permanent  magnets.  Thus  the  bar 
magnet  shown  in  Fig.  76,  has  its  magnetic 
circuit  completed  partly  through  the  bar 
and  partly  through  the  air  outside  the  bar. 


200  ELECTRICITY    IN 

A  very  common  type  of  aero-ferric 
magnetic  circuit  is  represented  in  Fig.  79, 
which  shows  an  electromagnet,  consist- 
ing essentially  of  a  bar  of  soft  iron  AB, 
wound   usually  with   a  large   number   of 


Fig.  79.— Bar  Electromagnet. 

turns  of  active  conductor.  Here  the  pres- 
ence of  the  iron  core  causes  the  flux  pro- 
duced by  the  current  passing  through  the 
coil,  to  be  more  powerful  than  that 
which  the  coil  alone  would  produce.  The 
polarity  of  the  iron  core  AB,  will,  of 
course,   depend   on    the   direction   of  the 


ELECTRO-THERAPEUTICS.  201 

current  in  the  wire.  If  this  direction  be 
such  that  the  flux  enters  the  core  at  the 
end  I>,  and  leaves  at  the  end  A,  then  the 
north  and  south  poles  of  the  electromagnet 
so  formed  will  be  as  marked  in  the  figure. 
The  introduction  of  the  iron  core,  has  not, 
therefore,  altered  the  polarity  produced  by 
the  helix,  but  it  has  greatly  increased  the 
quantity  of  magnetic  flux,  so  that  the 
magnet  exerts  a  greater  influence  at  a  dis- 
tance, and  also  a  greater  attractive  power 
at  its  poles.  Moreover,  when  the  core  is 
absent,  the  cessation  of  the  magnetizing 
current  is  accompanied  by  a  complete  loss 
of  the  magnetic  properties  of  the  coil ;  that 
is  to  say,  the  copper  wire  forming  the  coil 
possesses  no  permanent  magnetism.  If, 
however,  a  core  be  present,  the  magnet 
does  not  immediately  lose  its  magnetism 
on  the  cessation  of  the  current.  A  certain 
amount   of   flux   called   residual  flux,   or 


202  ELECTRICITY   IN 

■remanent  flux  remains  in  the  circuit. 
When  the  core  of  iron  is  very  soft  and 
carefully  annealed,  the  amount  of  this 
residual  magnetism  is  very  small.  When, 
however,  the  bar  is  formed  of  hard  iron, 
a  considerable  portion  remains  on  the 
cessation  of  the  magnetizing  current.  In 
the  case  of  a  bar  electromagnet,  the 
magnetic  circuit  is  largely  formed  of  air, 
less  than  half  of  the  circuit  existing  in  the 
iron  or  steel. 

If  we  bend  the  bar  shown  in  the  pre- 
ceding figure,  so  as  to  bring  the  two  poles 
nearer  together,  we  get  a  form  of  electro- 
magnet called  the  horse-shoe  electromagnet 
in  which  the  length  of  the  air  path  is  con- 
siderably reduced.  Instead  of  actually 
bending  the  bar,  it  may,  for  purposes  of 
convenience,  be  made  in  three  separate 
parts  as  shown  in  Fig.  80,  which  is  the 


ELECTRO-THERAPEUTICS.  203 

form  ordinarily  given  to  an  electromagnet. 
Here  the  magnet  consists  of  two  separate 
iron  cores,  connected  together  at  the  ends 
by  a  bar  of  soft  iron  called  a  yoke.  Each 
of  the  two  cores  is  provided  with  a 
magnetizing  coil. 


Fig.  80.— Electromagnet. 

The  value  of  the  electromagnet  depends 
largely  on  the  fact  that  its  core  being 
made  of  very  soft  iron,  possesses  the  prop- 
erty not  only  of  greatly  increasing  the 
strength  of  the  flux  produced  by  the  mag- 
netizing coils,  but  also  of  readily  losing 
nearly  all  its  magnetism  on  the  cessation  of 
the  magnetizing  current;  so   that   such  a 


204  ELECTRICITY   IN 

magnet  when  small  can  readily  acquire  and 
lose  its  magnetism,  many  times  in  a  second. 

Various  forms  are  given  to  electromag- 
nets according  to  the  purposes  for  which 
they  are  designed.  Where  it  is  desired 
that  an  electromagnet  should  possess  the 
power  of  attracting  or  repelling  magnetic 
bodies  at  a  considerable  distance  from  its 
poles,  the  circuit  is  necessarily  of  the  aero- 
ferric  type,  since  the  flux  must  pass  for  a 
considerable  distance  through  air;  but 
where  it  is  desired  that  the  magnet  shall 
possess  the  power  of  holding  heavy  weights 
attached  to  its  armature, — the  name  given 
to  the  bar  of  iron  completing  the  magnetic 
circuit, — this  circuit  approaches  more  nearly 
to  the  ferric  type. 

A  form  of  magnet  capable  of  producing 
very  powerful  magnetic  flux  in  the  space 


ELECTRO-THERAPEUTICS. 


205 


between  the    poles  is  shown  in   Fig.  81. 
It  consists  of  two  powerful   magnetizing 


yiA 

BH^HB 

flJ£ 

Fig.  81. — Electromagnet. 


coils   M,  M,   wound   on   iron   cores,   iron 
yoke  Yy  and  iron  pole  pieces  P,  P.     The 


206  ELECTRICITY   IN 

power  of  an  active  coil  to  produce  a  mag- 
netic flux  is  called  its  magnetomotive  force, 
generally  contracted  M.  M.  F.  This  mag- 
netomotive force  is  proportional  to  the 
number  of  turns  of  wire,  and  also  to  the 
current  strength  passing  through  the  same. 
Consequently,  if  we  add  more  turns  to  a 
coil,  or  increase  the  current  strength  pass- 
ing through  it,  we  will  increase  its  M.  M. 
F.,  and  produce  a  greater  magnetic  flux 
through  the  circuit. 

In  every  magnetic  circuit  the  strength 
of  the  magnetic  flux  depends  on  two 
quantities;  namely,  the  resistance  oppos- 
ing the  magnetic  flux,  called  the  magnetic 
resistance  or  reluctance.  A  similar  resist- 
ance to  the  passage  of  electrostatic  and 
electric  flux  exists  in  the  case  of  both  the 
electrostatic  and  electric  circuits.  The 
value  of  the  magnetic  flux,  like  that  of  the 


ELECTRO-THERAPEUTICS.  207 

electric   flux    may   be    expressed   by   the 

formula  of  Ohm's  law,  as  follows;  namely, 

_*  Magnetomotive  Force 

Magnetic    Flux  =        °  p  1 — 7 , 

&  Keluctance 

so   that  if   we  know  the  magnetomotive 

force  in  a  magnetic  circuit,  and  the  value 

of  the  magnetic  resistance  or  reluctance, 

by  dividing  the  former  by  the  latter, -we 

obtain  the  value  of  the  magnetic  flux. 

As  in  the  case  of  the  electric  circuit, 
special  names  are  given  to  the  unit  values 
of  these  quantities.  The  units  of  magneto- 
motive force  are  called  the  arrupere-t'ivrn,  and 
the  gilbert,  the  ampere-turn  being  greater 
than  the  gilbert  in  the  ratio  of  approxi- 
mately 5  to  4.  By  an  ampere-turn  is 
meant  the  amount  of  magnetomotive  force, 
which  is  produced  by  a  turn  of  wire  carry- 
ing a  current  of  one  ampere  ;  that  is  to  say, 
if  the  magnetizing  coils  shown  in  Fig.  80 


208  ELECTRICITY  IN 

coDsisted  of  200  turns  in  each  spool,  and  if  a 

current  of  five  amperes  passes  successively 

through   these   spools,  then   the  total  M. 

M.  F.,  urging  the  magnetic  flux  through  the 

circuit,  is  5  amperes  x  400  turns  =  2,000 

2,000X5         -k„       .,, 
ampere-turns  =   - — =  2,o00    gilberts, 

approximately.  The  amount  of  magnetic 
flux  produced  in  the  circuit  by  this  M.  M.  F. 
will  depend  entirely  upon  the  reluctance  of 
the  circuit.  If  the  air-gap  is  large ;  i.  e.9  if 
the  magnetic  circuit  contains  a  long  air  path, 
the  magnetic  resistance,  or  reluctance,  of  the 
circuit  will  be  great,  and  the  magnetic  flux 
produced  by  the  magnetomotive  force  will 
be  comparatively  small.  If,  on  the  other 
hand,  the  length  of  the  air-path  is  small, 
the  reluctance  will  be  very  small,  and  the 
amount  of  flux  produced  will  be  corre- 
spondingly great.  This  is  for  the  reason 
that  the  reluctance  of  iron  is  very  small  as 


ELECTRO-THERAPEUTICS.  209 

compared  with  that  of  air,  provided  that 
the  iron  is  not  saturated;  i.  e.,  is  not 
already  conducting  a  large  amount  of  flux 
per  square  inch  or  per  square  centimetre 
of  cross-sectional  area. 

As  in  the  electric  circuit,  the  resistance 
of  a  wire  depends  upon  its  length  and 
cross-sectional  area,  as  well  as  on  the 
nature  of  the  material  of  which  it  is  com- 
posed, so,  in  the  magnetic  circuit,,  the 
reluctance  depends  upon  the  length  and 
area  of  cross-section  of  the  circuit,  and  on 
the  nature  of  the  substance  through  which 
the  flux  is  passing.  In  order  to  decrease 
the  resistance  of  a  wire,  we  may  either 
decrease  its  length,  or  increase  its  area  of 
cross-section.  In  the  same  way,  in  order 
to  decrease  the  reluctance  of  a  magnetic 
circuit,  we  may  decrease  its  length  or 
increase   its    area   of    cross-section.       The 


210  ELECTRICITY  IN 

resistivity,  or  resistance  in  a  unit  cube, 
varies  markedly  with  the  nature  of  the 
substance,  but  does  not  vary  with  the 
current  strength  passing  through  the 
material,  provided  the  temperature  is  con- 
sidered as  remaining  the  same.  In  the 
magnetic  circuit,  the  reluctivity,  or  reluc- 
tance in  a  unit  cube  (reluctance  in  one 
cubic  centimetre  measured  between  par- 
allel faces)  is  practically  the  same  for  all 
substances  other  than  the  magnetic  metals ; 
in  which    the  reluctivity  is  much   lower. 

Unlike  the  case  of  the  electric  circuit, 
the  reluctivity  varies  markedly  with  the 
strength  of  the  magnetic  flax  passing 
through  the  circuit.  When  the  magnetic 
flux  passing  through  iron  is  feeble,  the 
reluctivity  may  be  a  thousand  times  less 
than  that  of  air,  while  iron  magnetically 
saturated ;    *.    e.,   carrying   a    very   dense 


ELECTRO-THER  Jp^JTKJSrt  L  F  L  £M  Y    (,  f 

magnetic  flux,  has  a  ikffetivity  prac- 
tically equal  to  that  of  air/V3£&^fiWtn!*rff 
reluctance  is  called  the  oersted,  and  is  eqtlaf" 
to  the  reluctance  offered  by  a  cubic  centi- 
metre of  air,  or  more  strictly  of  air-pump 
vacuum,  measured  between  parallel  faces, 
and  is,  therefore,  nearly  equal  to  the  reluc- 
tance of  a  cube  of  glass,  air,  wood,  copper, 
etc.,  measured  between  parallel  faces.  The 
reluctivity  of  air  is,  therefore,  taken  as 
unity. 

The  unit  of  magnetic  flux  is  called  the 
weber.  One  weber  will  flow  in  a  magnetic 
circuit  under  a  M.  M.  F.  of  one  gilbert, 
through  a  reluctance  of  one  oersted.  If 
the  reluctance  of  the  magnetic  circuit 
represented  in  Fig.  80,  be  0.5  oersted, 
then  since  its  M.  M.  F.  has  been  assumed 
at  2,500  gilberts,  the  flux  through  the  cir- 
cuit will  be  ^— -  —  5,000  webers. 

0.D 


212  ELECTRICITY   IN 

There  are  but  two  ways  of  varying  the 
M.  M.  F.  in  a  circuit ;  L  e.,  by  increasing 
the  number  of  turns,  or  by  increasing  the 
current  strength  circulating  in  them  ;  or, 
briefly,  by  increasing  the  number  of  am- 
pere-turns. 

It  is  necessaiy  to  draw  a  distinction 
between  the  total  flux  in  a  circuit  meas- 
ured in  webers,  and  the  intensity  of  the 
flux  per  unit  of  cross-sectional  area ;  /.  e., 
per  square  inch,  or  per  square  centimetre  ; 
just  as  it  is  necessaiy  to  distinguish 
between  the  total  current  strength  in  an 
electric  circuit,  as  measured  in  amperes, 
and  the  density  of  that  current,  as  meas- 
ured in  amperes-per-square-inch,  or  per- 
square-centimetre,  of  cross-sectional  area  in 
the  wire  conveying  the  current.  Since,  in 
the  case  of  the  magnetic  circuit,  the  intro- 
duction of  iron  is  invariably  attended  by 


ELECTRO-THERAPEUTICS.  213 

an  increase  in  the  magnetic  flux,  it  is 
evident  that  by  the  introduction  of  a  suf- 
ficiently great  amount  of  iron,  the  amount 
of  magnetic  flux  can  be  increased  almost 
to  any  extent.  Although  this  would  neces- 
sitate a  marked  increase  in  the  area  of 
cross-section  of  the  iron,  yet  the  flux  den- 
sity per  square  inch,  or  square  centimetre, 
may  not  be  increased.  Since  soft  iron 
practically  saturates  at  an  intensity  of 
19,000  webers-per-square-centimetre,  and 
its  reluctance  near  saturation  rapidly 
increases,  it  is  difficult  to  obtain  at  any 
portion  of  the  magnetic  circuit,  intensi- 
ties higher  than  .19,000  webers-per-square 
centimetre  ;  i.  e.,  19,000  gausses,  the  gauss 
being  the  wn/it  of  magnetic  intensity,  or 
the  density  of  one  weber-per-square-centi- 
metre  of  perpendicular  area  of  cross- 
section.  The  intensity  of  the  earth's  mag- 
netic flux  is,  approximately,  half  a  gauss, 


214  ELECTRICITY   IN 

while  the    highest  experimental  intensity 
on  record  is  45,350  gausses. 

It  does  not  appear  that  magnetic  flux 
produces  any  apparent  physiological  ef- 
fects on  the  human  body.  The  human 
body,  containing  in  its  composition  no 
appreciable  quantity  of  magnetizable  ma- 
terial, has  practically  the  same  reluc- 
tivity as  ordinary  air;  that  is  to  say, 
the  interposition  of  the  human  body  in 
a  magnetic  circuit  does  not  appreciably 
affect  the  distribution  of  the  magnetic 
flux.  For  example,  if  a  delicately  sus- 
pended magnetic  needle  be  deflected  by 
a  magnet  placed  at  a  certain  distance  from 
it,  the  direct  interposition  of  the  body  of 
a  person  between  the  magnet  and  needle 
is  not  found  to  produce  any  appreciable 
effect,  although  if  the  same  person  wears, 
for    example,    an    iron    rimmed    pair    of 


ELECTRO-THERAPEUTICS.  215 

spectacles,  or  carries  a  key  in  his  pocket, 
the  effect  on  the  needle  may  be  very 
marked.  This  is  because  the  magnetic 
ilux,  acting  on  the  needle,  passes  through 
the  body  of  the  person  as  readily  as 
through  the  previously  intervening  air, 
but  the  magnetic  influence  of  the  iron 
rimmed  spectacles,  or  the  key,  may  have 
a  powerful  influence  on  a  delicately  sus- 
pended needle  even  though  twenty  feet 
away  from  it. 

Not  only  is  the  reluctivity  of  the  human 
body  practically  the  same  as  that  of  other 
non-magnetic  materials,  but  portions  of 
the  body  subjected  to  powerful  magnetic 
fluxes  do  not  appear  to  have  produced  in 
them  any  appreciable  physiological  effects. 
Although  experiments  are  still  wanting 
concerning  the  physiological  influence 
which  long   sustained  powerful  magnetic 


216  ELECTRICITY   IN 

flux  may  exert,  yet  it  has  been  shown  that 
human  beings  and  dogs  subjected  for 
many  minutes  to  intensities  of  magnetic 
flux  of  about  2,500  gausses,  and,  therefore, 
about  5,000  times  that  of  the  earth's 
magnetic  flux,  have  not  experienced  any 
influence  that  could  be  observed.  Simi- 
larly, experiments  made  both  with  con- 
tinuous and  rapidly  alternating  magnetic 
fluxes  have  not  shown  any  effect  produced 
upon  the  circulation  of  the  blood  due  to 
the  iron  it  contains,  upon  ciliary  or  proto- 
plasmic movements,  upon  sensory  or  motor 
nerves,  or  upon  the  brain. 

It  has  been  positively  asserted  that  in 
a  perfectly  dark  room  certain  individuals 
possess  the  power  of  observing  faint 
luminous  phenomena,  around  the  poles  of 
permanent  or  electro-magnets ;  that  is,  that 
these  persons  actually  possess  the  power 


ELECTRO-THERAPEUTICS.  217 

of  being  visually  affected  by  magnetic 
flux.  Investigations,  however,  have  not 
only  thrown  doubt  upon  the  original  ex- 
periments, but  repetitions  of  these  experi- 
ments, with  powerful  electromagnets,  have 
entirely  failed  to  confirm  the  statements. 

So  far,  therefore,  as  we  know  at  the 
present  time,  it  would  appear  that  mag- 
netic flux  is  absolutely  without  influence 
either  upon  the  human  body,  or  on  any  of 
its  physiological  processes,  and  that,  conse- 
quently, if  any  therapeutic  effects  do 
attend  the  use  of  magnets,  the  causes  must 
be  of  a  psychic  rather  than  of  a  physio- 
logical nature.  It  is  to  be  remembered, 
however,  that  carefully  conducted  re- 
searches with  very  powerful  magnetic 
fluxes  may  yet  show  lesser  residual  influ- 
ences, which  the  experiments  up  to  the 
present  time  have  failed  to  bring  to  light. 


218  ELECTRICITY    IN 

But  up  to  the  present  time  experiments 
made  on  human  beings  have  failed  to 
establish  any  physiological  effects  what- 
ever, even  when  such  a  delicate  organ  as 
the  brain  is  placed  in  the  direct  passage  of 
a  powerful  magnetic  flux.  When,  for  ex- 
ample, a  person  is  placed  with  his  head 
between  the  poles  of  a  powerful  dynamo- 
electric  machine,  from  which  the  armature 
has,  been  removed,  so  that  the  flux  passes 
directly  through  the  head,  even  prolonged 
exposure  has  failed  to  produce  any  ob- 
served effect  either  on  the  pulse  or  respira- 
tion, whether  the  magnetic  flux  was  inter- 
mittent or  was  steadily  maintained. 

Or,  take  the  case  of  a  powerful  electro- 
magnet, made  by  wrapping  an  iron  cannon 
with  a  suitable  magnetizing  coil,  and  pro- 
ducing a  flux  sufficiently  great  to  cause 
heavy  iron  bars  or  bolts  to  be  sustained  on 


ELECTRO-T  Fl  ER  APEUTICS 


219 


the  person  of  a  soldier  standing  before  the 
gun,  as  shown   in   Fig.  82.     Under  these 


Fig.  82. — Magnetic  Gun  Attraction  through  a 
Soldier's  Body. 


circumstances,  no  sensations  were  experi- 
enced by  the  soldier  other  than  those  of 
pressure  from  the  attracted  masses  of  iron. 


220  ELECTRICITY. 

It  would  appear  evident  from  the  pre- 
ceding  observations   that   very   little  cre- 
dence can  be  placed   on  the   extravagant 
claims  as  to  the  curative  power  possessed 
by  small  magnets  carried  or  worn  on  the 
body.     The   magnetic    flux   produced    by 
such  magnets  is  necessarily  comparatively 
feeble,  and   if   the   more   powerful  fluxes 
before  referred  to  failed   to   produce  any 
appreciable  physiological  effects,  there  are 
no  reasons  for  believing  that  these  feeble 
fluxes   can    produce    any    marked    effects 
unless  that  due  to  a  feeble  influence,  long 
sustained.     In  the  case,  however,  of  most 
of  the  magnetic  nostrums,  for  which  cura- 
tive effects  are  claimed,  even  the  weak  flux 
they  produce  usually  fails  to  be  properly 
directed,  does  not  pass  through  any   por- 
tion of  the  body,  and  can,  therefore,  have 
no  physiological  effect,  except  through  the 
medium  of  the  imagination. 


CHAPTER  IX. 

INDUCTION    OF    E.    M.    F.    BY    MAGNETIC    FLUX. 

When  a  conducting  loop  is  filled  with, 
or  emptied  of,  magnetic  flux,  electromotive 
forces  are  thereby  set  up  or  induced  in  the 
loop.  This  is  called  the  induction  of  M 
M.  F.  by  magnetic  flux.  Four  cases  of 
such  induction  may  arise  ;  namely, 

(1)  Self  induction. 

(2)  Mutual  induction. 

(3)  Electro-magnetic  induction. 

(4)  Magneto-electric  induction. 

We  have  seen  that  when  an  electric 
current  circulates  through  a  coil  or  loop, 

221 


222  ELECTRICITY    IN 

all  the  flux  produced  by  the  current  is 
caused  to  enter  the  loop  at  one  face,  and  to 
emerge  at  the  opposite  face.  When  a  cir- 
cuit is  closed,  so  that  the  electric  source 
begins  to  force  electric  currents  through 
the  circuit  connected  with  it,  some  little 
time  is  required  before  the  full  current 
strength  is  established ;  so  that,  during 
this  time,  the  magnetic  flux  that  is  passing 
through  the  loop  is  increasing  in  strength. 
Also,  when  the  circuit  is  opened,  some 
time  is  required  for  the  current  to  entirely 
cease  flowing  through  the  circuit,  and,  dur- 
ing this  time,  the  magnetic  flux  passing 
through  the  loop  is  decreasing.  Therefore, 
both  at  the  moment  of  making  and  break- 
ing an  electric  circuit,  a  tendency  will 
exist,  if  the  circuit  contains  coils  or  con- 
ducting loops,  for  electromotive  forces  to 
be  induced  in  the  circuit.  These  E.  M.  Fs. 
continue  only  while  the  current  strength  is 


ELECTRO-THERAPEUTICS.  223 

varying;  as  soon  as  the  current  strength 
in  the  circuit  becomes  constant,  they  dis- 
appear. 

The  amount  of  the  E.  M.  F.  induced 
at  any  moment  of  time  in  a  conducting 
loop,  by  filling  or  emptying  it  with  flux, 
depends  upon  the  rate  at  which  the  loop 
is  filled  with,  or  emptied  of  flux.  Sup- 
pose, for  example,  that  100,000  webers  are 
passed  through  a  loop,  in,  say  two  seconds 
of  time:  then  if  the  rate  at  which  this 
flux  enters  the  loop  is  uniform,  the  E.  M. 
F.  generated  in  the  'loop  will  be  main- 
tained during  the  entire  two  seconds,  and 
will    be    equal    to  the    rate  of    entry  in 

i                        ,            200,000 
webers-per-second,    or    1 =  100,000 

webers-per-second  =  100,000  units  of  E.  M. 
F.  The  unit  of  E.  M.  F.,  the  volt,  has  been 
so  chosen  that  100,000,000  webers,  passing 


224  ELECTRICITY  IN 

through  the  loop  per  second,  generate  one 
volt;   so  that  this  E.  M.  F.  is  100,000  H- 

100,000,000  =_L-volt.        If,    however, 

the  200,000  webers,  above  mentioned, 
entered  the  loop  in  say  0.01  of  a 
second,  the  E.  M.  F.  induced  in  the  loop 

would  be  200  times  greater,  or 1 —  = 

8  0.01 

20,000,000  units  of  E.  M.  F.  =  0.2  volts, 
but  this  E.  M.  F.  would  only  last  for  the 
1/1 00th  of  a  second.  When,  therefore,  a 
loop  is  filled  with  and  emptied  of  a  given 
number  of  webers  of  flux,  the  E.  M.  F. 
which  will  be  produced  in  the  loop  de- 
pends entirely  upon  the  time  in  which  the 
filling  and  emptying  takes  place.  If  the 
filling  takes  place  very  suddenly,  the  E. 
M.  F.  will  be  powerful,  but  of  very  short 
duration.  On  the  other  hand,  if  the  fill- 
ing  or  emptying   takes  place  slowly,   the 


ELECTRO-THERAPEUTICS.  225 

E.  M.  F.  will  be  correspondingly  weaker, 
but  longer  sustained. 

The  direction  of  the  E.  M.  F.  induced 
by  filling  a  conducting  loop  with  flux,  is 
opposite  to  that  induced  by  emptying  the 
same  loop  of  flux.  The  direction  of  the 
E.  M.  F.  induced  by  filling  a  conducting 
loop  with  flux,  is  readily  remembered  by 
the  following  rule : 

Regarding  the  loop  as  the  face  of  a 
watch,  held  in  front  of  the  observer,  then 
if  the  flux  passes  through  the  loop  in  the 
same  direction  as  the  light  passing  from 
the  face  of  the  watch  to  the  observer's 
eye,  the  E.  M.  F.  induced  in  the  loop  will 
have  the  same  direction  as  that  of  the 
hands  of  the  watch. 

Some  general  idea  concerning  the 
manner  in  which  E.  M.  F.  is  generated  in 


226 


ELECTRICITY   IN 


a  loop  by  the  passage   of   magnetic   flux 
through  it,  may,  perhaps,  be  obtained  from 


Fig.  83.— Mechanical  Model  Having  Analogies  with 
Electric  Circuit. 


the  mechanical  model  shown  in  Fig.  83. 
A  cylinder  AB,  pivoted  upon  a  vertical 
axis  CD,  mechanically  represents  a  con- 
ducting loop  of  wire.     The  cylinder  is  con- 


ELECTRO-THERAPEUTICS.  227 

nected  with  the  axis  by  a  number  of  radial 
spokes  in  the  form  of  fan-blades,  so  that, 
if  a  stream  of  liquid,  such  as  water,  be 
poured  through  the  cylinder  or  loop  from 
above,  the  impact  of  the  water  on  the 
blades  will  cause  the  cylinder  to  rotate  in 
a  direction  opposite  to  that  of  the  hands  of 
a  watch.  If,  however,  the  water  be  forced 
upward  through  the  loop,  its  impact  will 
cause  the  cylinder  to  rotate  in  the  oppo- 
site direction.  If  a  given  number  of  gal- 
lons of  water  be  passed  through  the 
cylinder,  the  driving  impulse  communicated 
to  it  will  depend  upon  the  time  during 
which  the  water  passes.  If  the  water  be 
delivered  in  a  brief  time,  its  rate  of  pas- 
sage through  the  loop  will  be  great,  and 
the  driving  impulse  communicated  to  the 
cylinder  will  be  great,  though  of  brief 
duration.  If,  on  the  other  hand,  the  time 
during  which  the  water  passes  through  the 


228  ELECTRICITY    IN 

loop  be  considerable,  the  driving  impulse 
exerted  on  the  cylinder  will  be  prolonged, 
but  correspondingly  feeble.  It  will  be 
observed  that  the  driving  impulse  or  force 
in  this  mechanical  analogue  stands  for 
electromotive  force  in  the  electrical  case. 
When  the  water  is  first  poured  through 
the  cylinder,  the  inertia  of  the  cylinder  will 
prevent  it  from  being  immediately  set  in 
motion.  Similarly,  when  the  water  has 
ceased  to  pass,  the  motion  of  the  cylinder, 
owing  to  inertia,  does  not  immediately- 
cease.  In  the  electric  circuit,  this  corre- 
sponds to  the  effect  of  self-induction  ;  for, 
the  effect  of  pouring  flux  into  a  loop  is  to 
induce  in  the  loop  an  E.  M.  F.,  and  the 
effect  of  the  current  so  set  up,  is  to  pro- 
duce in  the  loop  a  flux  opposite  to  that 
of  the  inducing  flux,  producing  thereby 
a  C.  E.  M.  F.  retarding  the  development 
of   the   electric    current.      On    the   other 


ELECTRO-THERlt^BUTICSn  C  P  ijffif  Y    f  l 

hand,  when  the  flux  has \ffifecl   the   loop, 
the   current   does   not   imniMi^^^ftcea^e  r  > m n 
flowing,  being  prolonged  by  the  ac^iofTTS^ 
the  flux  set  up  by  the  current ;  in  other 
words,  the  loop  acts  as  though  it  possessed 
electrical  inertia. 

When  a  number  of  turns  are  connected 
in  series,  as,  for  example,  in  the  case  of  the 
coil  of  conducting  wire  shown  in  Fig.  77, 
the  effects  produced  by  each  turn  are 
added,  so  that  the  coil  has  induced  in  it  an 
E.  M.  F.  proportional  to  the  number  of  its 
turns.  When  a  conducting  coil,  contain- 
ing many  turns,  has  its  terminals  connected 
to  a  voltaic  battery,  some  little  time 
elapses  before  the  full  current  strength  is 
established  in  the  circuit.  The  reason  is 
to  be  found  in  the  C.  K  M.  F.  of  self 
induction  of  the  coil.  Similarly,  when  the 
circuit  of  this  coil  is  opened,   the  current 


230  ELECTRICITY   IN 

does  not  instantly  cease  flowing  through 
it, -since  the  emptying  of  the  coil  of  the 
flux,  produces  in  it  an  E.  M.  F.  which  es- 
tablishes in  the  coil  a  current  in  the  same 
direction  as  that  sent  through  it  by  the 
battery.  In  other  words,  the  E.  M.  F. 
produced  in  the  coil  by  self-induction,  at 
the  moment  of  making,  tends  to  oppose  the 
establishment  of  the  current,  and  that  pro- 
duced at  the  moment  of  breaking,  tends  to 
aid  the  passage  of  the  current. 

When  the  circuit  of  an  electric  source, 
such  as  a  voltaic  battery,  is  opened,  a 
minute  spark  is  frequently  visible  at  the 
point  of  opening.  If,  however,  a  coil  of 
many  turns  of  wire  be  contained  in  the  cir- 
cuit, the  spark  upon  opening  the  circuit 
will,  probably,  be  much  greater,  and  a  dis- 
tinct shock  may  be  felt  under  favorable 
conditions  by  the  person  opening  the  cir- 


ELECTRO-THERAPEUTICS.  231 

cuit.  This  spark  is  due  to  the  self-induc- 
tion of  the  circuit.  The  current  which  has 
passed  through  the  circuit  has  produced  a 
magnetic  flux  linked  with  the  turns ;  i.  e., 
the  loops  in  the  coil  or  coils  of  wire.  On 
the  opening  of  the  circuit,  this  current  is 
suddenly  interrupted,  and  the  flux,  rapidly 
disappearing  from  the  coils  ;  i.  e.  pouring 
out  of  them,  induces  a  brief  but  powerful 
E.  M.  F.,  which,  actiug  in  the  same  direc- 
tion as  the  current,  tends  to  prolong  it. 

If  two  coils  A  and  £,  Fig.  84,  connected 
in  separate  circuits,  are  placed  side  by  side, 
and  an  electric  current  be  sent  through 
one,  say  A,  the  passage  of  this  current  will 
produce  a  magnetic  flux,  part  of  which  will 
pass  through  B.  During  the  process  of 
filling  J3,  with  this  portion  of  A'a  flux,  an 
E.  M.  F.  will  be  set  up  in  each  turn  of  £, 
equal,  at  any  moment,  to  the  rate  at  which 


232 


ELECTRICITY   IN 


the  flux  is  entering  in  webers-per-second ; 
or,  expressed  in  volts,  to  the  rate  at  which 
the  flux  is  entering  in  hundred  millions  of 
webers-per-second.      As   soon  as  the 


cur 


Fig.  84.— Diagram  Illustrating  Mutual  Induction. 


rent  in  A,  becomes  stationary,  the  flux 
through  B,  due  to  this  current,  is  also 
stationary,  and,  consequently,  no  further 
E.  M.  F.  is  induced  in  B.  If,  however,  the 
current  strength  in  A,  diminishes,  its  flux 
through  B,  will  be  correspondingly  dimin- 
ished, and  an  E.  M.  F.  will  be  induced  in 


ELECTRO-THERAPEUTICS.  233 

B,  in  the  opposite  direction  to  that  origin- 
ally produced,  and  equal  in  volts  to  the 
rate  of  emptying  in  millions  of  webers-per- 
second.  When  the  flux  through  J3,  due  to 
the  current  in  A,  has  entirely  disappeared, 
the  E.  M.  F.  induced  in  jB,  has  also  disap- 
peared. If  there  be  100  turns  of  wire  in 
the  coil  B,  the  E.  M.  F.  induced  in  the  coil 
will  be  100  times  as  great  as  if  it  consisted 
of  a  single  turn,  assuming  that  the  same 
quantity  of  A1  a  flux  passes  through  all  of 
B's  turns  alike.  This  inductive  influence 
extending  from  one  coil  to  another,  whereby 
a  current  in  one  circuit  induces  an  E.  M.  F. 
in  another  circuit,  is  called  mutual  induction. 
An  example  of  mutual  induction  can  be 
shown  by  means  of  the  apparatus  repre- 
sented in  Fig.  85,  in  which  A,  represents 
the  inducing  coil ;  L  e.,  the  coil  in  which 
the  current  flows;  and  B,  the  coil  in  which 
the   current  is  induced.     Or,  as   they  are 


234 


ELECTRICITY   IN 


generally  called,  A,  is  the  primary  coilaxiA 
JB,  is  the  secondary  coil.  If  the  terminals 
of  the  primary  coil  A,  be  connected  with 


Fig.  85.— Mutual  Induction. 

the  voltaic  cell  C\  as  shown  in  the  figure, 
and  the  terminals  of  the  secondary  coil  B, 
be  connected  with  an  ammeter,  or  galva- 
nometer, G)  then,  as  soon  as  the  current 


ELECTRO-THERAPEUTICS.  235 

is  established  in  A,  no  current  will  be 
induced  in  B,  as  long  as  A,  remains  at 
rest.  If,  however,  A,  be  moved  either 
toward  or  from  B,  currents  will  be  pro- 
duced in  the  secondary  coil,  as  will  be  indi- 
cated by  the  galvanometer,  the  current 
passing  iu  one  direction,  when  A,  is  moved 
toward  B,  and  in  the  opposite  direction 
when  A,  is  moved  from  B.  It  can  be 
shown  that  the  current  induced  in  a 
secondary  coil  is  induced  in  the  opposite 
direction  to  that  in  its  primary,  on  the 
approach  of  A  to  B,  and  in  the  same  di- 
rection, on  its  withdrawal  from  B.  The 
two  circuits  A  and  By  although  electrically 
disconnected,  are  connected  magnetically 
by  the  flux  permeating  the  space  between 
them,  and  the  E.  M.  F.  of  mutual  induction 
is  caused  by  the  flux  proceeding  from  the 
primary  coil  being  carried  toward  or  from 
the  secondary  coil,  during  its  motion,  so  as 


236  ELECTRICITY   IN 

to  cause  the   secondary   coil   to  be   filled 
with  more  or  less  flux. 

That  mutual  induction  may  take  place 
between  stationary  primary  and  secondary 
coils,  may  be  experimentally  demonstrated. 
For  example,  if  as  in  Fig.  86,  the  primary 
coil  A,  is  fixed  at  a  constant  distance  from 
£y  then  on  completing  the  circuit  of  the 
primary  coil,  by  closing  the  switch  S, 
while  the  current  is  increasing  in  the  pri- 
mary, the  magnetic  flux  produced  by  it 
passes  through  the  conducting  loops  on  the 
secondary  coil  C,  thereby  inducing  an 
E.  M.  F.  and  establishing  a  current,  as  is 
shown  by  the  galvanometer  G. 

The  distinction  between  electro-magnetic 
and  magneto-electric  induction  is  seen  in 
Fig.  87,  where  the  motion  of  the  magnet 
M,  into  or  out   of  the   coil  of  wire,  pro- 


ELECTRO-THERAPEUTICS.  237 

duces  electromotive  forces  in  the  coil  C, 
as  shown  by  the  galvanometer  G.  When 
the   magnet   is   thrust   into   the    coil,  the 


Fig.  86.— Mutual  Induction. 

galvanometer  indicates  a  temporary  cur- 
rent in  one  direction,  and,  on  its  with- 
drawal from  the  coil,  it  shows  a  current 
in  the  opposite  direction.     The  introduc- 


238  ELECTRICITY   IN 

tion  of  the  south  pole  into  the  coil  pro- 
duces the  same  direction  of  current  as 
the  withdrawal  of  the  north  pole.  Here, 
as  in  the  other  instances,  E.    M.   Fs.  are 


Fig.  87. — Magneto-Electric  Induction. 

induced  by  the  passage  of  magnetic  flux 
through  the  coil ;  the  flux  produced  by  the 
magnet,  being  advanced  or  moved  so  as  to 
pass  through,  or  link  with,  the  turns  in  the 
secondary  coil. 

A    form    of    apparatus    for    producing 
E.  M.  Fs.  by  magneto-electric  induction  is 


ELECTRO-THERAPEUTICS. 


239 


represented    in    Fig.    88.     A    permanent 
horse-shoe   magnet,  MM,  is  supported  in 


-  Elee.  World 

Fig.  88.— Magneto-Electric  Generator. 


a  vertical  position,  and  two  coils  of  fine 
insulated  wire  GG,  are  supported  on  a 
horizontal  axis,  in  such  a  manner  as  to  be 
capable  of  rotation  by  the  turning  of  the 


240  ELECTRICITY   IN 

handle  JT,  the  rotary  speed  of  the  coils  be- 
ing made  greater  than  the  rotary  speed  of 
the  handle,  by  the  interposition  of  suitable 
multiplying  gear.  The  coils  are  wound 
on  cores  of  soft  iron,  which  are  connected 
by  a  soft  iron  yoke  y.  The  ends  of  the 
cores  revolve  in  close  proximity  to  the 
poles  of  a  permanent  magnet,  leaving  a 
small  air  gap  or  clearance  of  compara- 
tively small  reluctance.  When  the  two 
coils  stand  vertically,  the  flux  from  the 
magnet  passes  through  the  air  gap,  the 
cores  and  their  connecting  yoke,  thereby 
filling  all  the  turns  of  wire  wound  upon 
the  core.  An  E.  M.  F.  will  be  induced  in 
each  turn  equal  in  volts  to  the  rate  of  fill- 
ing it  with  flux,  in  hundred  millions  of 
webers-per-second  ;  and,  since  all  the  turns 
in  each  coil,  and  the  two  coils  themselves, 
are  connected  in  series,  the  total  E.  M.  F. 
will  be  correspondingly  multiplied. 


ELECTRO-THERAPEUTICS. 


241 


The  condition  of  affairs  in  the  preceding 
machine,  is  represented  in  Fig.  89,  where 


Fig.  89.— Diagram  Representing  Changes  in  the  Mag- 
netic Circuit  of  Magneto-Electric  Generator. 


the  coils  are  shown  at  A,  as  being  imme- 
diately opposite  to  the  magnet  poles,  and 


242  ELECTRICITY  IK 

in  such  a  position  as  to  be  filled  with  flux, 
so  that  they  cannot  receive  any  further 
increase  of  flux  by  a  further  rotation  in 
either  direction. 

At  £,  the  coils  are  leaving  the  pole 
pieces,  so  that  the  reluctance  in  the 
magnetic  circuit  is  increasing,  and  the 
magnetic  flux,  which  passes  through  the 
cores  of  the  coils,  is  diminishing.  In 
other  words,  the  coils  are  becoming 
emptied  of  the  flux  they  contain.  An 
E.  M.   F.  is,  therefore,  induced  in  them. 

At  C%  the  coils  are  completely  emptied 
of  magnetic  flux,  and,  therefore,  have  no 
E.  M.  F.  At  Z>,  the  coils  are  being  filled 
with  flux,  but  in  the  opposite  direction 
to  that  which  exists  at  A.  Consequently, 
the  E.  M.  F.  induced  has  the  opposite 
direction  to  that  induced  at  B. 


ELECTRO-THEKAPEUTICS.  243 

At  jS7,  the  coils  are  full  of  flux  in  the 
opposite  direction  to  that  at  A.t  and  the 
E.  M.  F.  in  the  coils  will  have  ceased. 

It  will,  therefore,  be  evident  that  dur- 
ing any  half  revolution,  as  from  A  to  E, 
the  E.  M.  F.  induced  in  the  coils  has  made 
a  single  alternation  or  reversal ;  and  that 
during  the  next  succeeding  half  revolution, 
in  which  the  coil  returns  to  the  position  A, 
the  E.  M.  F.  induced  will  be  of  the  same 
magnitude  as  above  pointed  out,  but  in  the 
opposite  direction. 

The  revolving  coils,  therefore,  generate 
alternating  currents  in  the  circuit  con- 
nected with  them,  the  E.  M.  F.  being 
alternately  in  opposite  directions,  during 
successive  half  revolutions.  One  com- 
plete revolution  of  the  coils  produces  one 
complete   double  alternation,    or   cycle   of 


244  ELECTRICITY   IN 

the  E.  M.  F.  and  electric  current,  con- 
sequently, the  frequency  of  the  alternat- 
ing currents  produced;  i.  e.,  the  number 
of  complete  double  alternations,  or  cycles 
per    second,    is    equal  to   the  number   of 


Fig.  90.— Diagram  of  a  Possible  Wave  Form  of  Mag- 
neto-Electric Generator  E.  M.  F. 

revolutions  made  by  the  coils  per  second. 
The  alternating  E.  M.  F.  produced  by 
this  machine  might  be  represented  dia- 
grammatical ly  in  Fig.  90.  The  exact  wave 
form,  in  each  case,  would  depend  upon  the 
shape  of  the  poles  and  of  the  iron  cores. 
If,  however,  a  commutator  be  employed  on 
the  armature,  as  shown  in  Fig.  91,  whereby 


^ 


{* 


^  &  WBFtflS 


ELECTRO-THER; 


"WC/fifty  U 


at  each  half  revolution  flj|te6£onnections  of 


v*. 


is  c 


the  coils  with  the  extern 

versed,  the  current  produced 

nating  E.  M.  F.  will  be  unidirectional  in 

the  external  circuit. 


is   re-        ^9. 


Fig.  91.— Diagram  op  Two-Part  Commutator. 


Fig.  92,  represents  the  corresponding 
form  of  pulsating  E.  M.  F.  wave  produced 
in  the  external  circuit  when  the  commu- 
tator is  employed.  It  will  be  seen  that 
the  E.  M.  F.  is  now  always  above  the  line. 
If  the  E.  M.  F.  were  reversed,  the  waves 


246  ELECTRICITY   IN 

might    be   represented    as   being   entirely 
below  the  line. 

Fig.  93,  represents  a  form  of  magneto- 
electric  machine,  the  current  from  which 
is    capable    of    lighting    a    small    incan- 


Fig.  92.— Diagram  of  a  Possible  Wave  Form  of  Mag- 
neto-Electric Generator  E.  M.  F.  (when  a  Com- 
mutator is  Employed). 

descent  lamp.  If  an  alternating  mag- 
neto-electric generator,  that  is  a  magneto- 
electric  generator  not  employing  a  commu- 
tator, is  connected  to  the  body  of  a  patient 
by  suitable  electrodes,  alternating  electric 
currents  will  pass  through  the  body.  If, 
however,  a  commutator  be  employed,  and 
the  currents  be  of  the  wave  form  shown  in 
Fig.  92,  the  physiological   effects  will  be 


ELECTRO-THERAPEUTICS.  247 

somewhat  different.  The  type  of  current 
of  Fig.  93,  possesses  polar  properties ;  i.  e.y 
possesses    the   characteristics   of   unidirec- 


Fig.  93. — Magneto-Electric  Generator. 

tional  currents,  while  symmetrical  alterna- 
ting currents  do  not  possess  these  proper- 
ties, since  the  polar  effects  produced  by  one 
wave,  are  neutralized  by  the  following 
wave  in  the  opposite  direction. 


CHAPTER  X. 

THE    MEDICAL    INDUCTION    COIL. 

The  medical  induct  ton  coil,  generally 
called  the  faradic  coil,  is  very  frequently 
employed  in  electro-therapeutics.  It  con- 
sists essentially  of  means  whereby  E.  M.  Fs. 
are  induced  by  mutual  induction,  and,  con- 
sequently, of  a  primary  and  a  secondary  cir- 
cuit. A  simple  form  of  induction  coil  is  rep- 
resented in  Fig.  94,  where  the  terminals  of 
the  primary  circuit  are  shown  at  P,  Py  and 
those  of  the  secondary  circuit  at  S,  S.  Fig. 
95,  shows  a  similar  coil  in  longitudinal  sec- 
tion. An  inspection  of  the  latter  figure  will 
show  that  the  primary  coil  P,  consists  of  a 
comparatively  short  length  of  fairly  coarse 

248 


ELECTRO-THERAPEUTICS.  249 

wire,  wrapped  around  a  hollow  bobbin. 
The  secondary  circuit  generally  consists  of  a 
greater  length  of  finer  fire  wrapped  either 
directly  over  the  secondary,  or  on  a  hollow 
bobbin  capable  of  being  moved  over  the 


Fig.  94.— Simple  Form  of  Induction  Coil. 

primary.  When  the  current  strength  in  the 
primary  circuit  is  varied  ;  i.  e.,  when  either 
an  alternating  or  a  pulsatory  current  is  sent 
through  the  primary,  an  alternating  E.  M. 
F.  is  induced  in  the  secondary  circuit  by 
the  influence  of  mutual  induction. 

The  amount  of  E.  M.  F.  induced  in  the 
secondary  circuit  depends  upon  the  num- 
ber  of   turns  in  its  coil  and  the  rate  at 


250  ELECTRICITY   IN 

which  the  magnetic  flux  fills  and  empties 
these  turns.  The  induced  E.  M.  F.  does 
not  depend  upon  the  number  of  yards  or 
feet  of  wire  in  the  secondary  coil,  except 
in  so  far  as  a  greater  length  of  wire  pro- 


's * 

Fig.  95.— Section  of  Simple  Form  of  Induction  Coil. 

vides  a  greater  number  of  turns  in  the  coil. 
If  we  double  the  number  of  turns  in  the 
coil  without  altering  in  any  way  the 
amount  of  flux  which  passes  through  each 
turn,  we  double  the  number  of  volts  in- 
duced therein ;  whereas,  if  we  double  the 
number  of  feet  or  yards  in  the  secondary 
coil,  we  do  not  necessarily,  and  in  point  of 
fact  very  rarely,  double  the   number  of 


ELECTRO-THERAPEUTICS.  251 

turns,  and,  therefore,  the  number  of  volts, 
because  the  average  length  of  turn  in  each 
successive  layer  increases.  If  we  double 
the  rate  at  which  the  flux  threads  or  links 
with  the  turns  of  the  secondary  coil,  we 
double  the  E.  M.  F.  induced  in  the  coil. 

An  increased  rate  of  filling  and  emptying 
conducting  loops  or  turns  with  flux  can 
be  obtained  in  one  or  both  of  two  ways ; 
viz., 

(1)  By  causing  the  same  flux  to  fill  and 
empty  the  loop  a  greater  number  of  times 
per  second  ;  i.  e.,  increasing  the  frequency 
of  oscillation  of  the  flux  in  the  magnetic 
circuit;  and, 

(2)  By  increasing  the  amount  of  mag- 
netic flux  in  the  circuit  without  increasing 
the  frequency  of  oscillation,  so  that  more 
flux  enters  or  fills  the  coils  at  each  alterna- 
tion. 


ELECTRICITY    IN 

Consequently,  for  a  given  primary  and 
secondary  circuit,  with  a  given  geometrical 
relationship  between  them,  we  can  only 
increase  the  E.  M.  F.  in  the  secondary 
circuit  either  by  increasing  the  magnetic 
flux,  or  by  increasing  the  frequency  of  flux 
oscillation,  or  both. 

In  order  to  increase  the  frequency  of 
flux  oscillation,  we  require  to  increase  the 
frequency  of  the  primary  current.  On  the 
other  hand,  in  order  to  increase  the  total 
amount  of  flux  we  must  either  increase  the 
M.  M.  F.  of  the  primary  circuit,  or  diminish 
the  reluctance  of  the  magnetic  circuit ; 
that  is  to  say,  we  must  either  employ 
more  ampere-turns  at  the  same  frequency, 
or  employ  such  a  form  of  iron  core  in  the 
primary  coil  as  will  increase  the  magnetic 
flux  from  a  given  M.  M.  F.  by  diminish- 
ing the  magnetic  resistance  of  its  circuit. 


ELECTRO-THERAPEUTICS. 


253 


The  frequency  required  for  the  primary 
circuit  may  be  obtained  either  by  an  al- 
ternating, or  by  a  continuous,  but  pulsat- 
ing   current.     The    ordinary   farad ic   coil 


Fig.  96.— Primary  Connections  of  Medical  Induction 
Coil. 


only  employs  the  latter,  the  connections 
being  represented  in  Fig.  96.  As  soon  as 
the  circuit  is  closed  at  the  switch  W,  the 
current  flows  through  the  primary  coil  of 
the  instrument,  into  the  spring  p,  through 


254 


ELECTRICITY  IN 


the  contact  c,  and  the  screw  stud  S,  in  the 
support  T.  The  M.  M.  F.  of  this  current 
produces  a  magnetic  flux  passing  through 
the  core,  and  through  the  air  outside,  in 
circuital  paths.  This  flux  being  produced 
within  the  magnetic  circuit,  sets  up  a  C. 
E.  M.  F.  of  self-induction,  tending  to  re- 
tard the  development  of  both  flux  and 
current  in  the  primary  coil,  so  that  the 
primary  current  does  not  instantly  reach 
its  full  strength,  but  rises  comparatively 
slowly  to  a  maximum.  As  soon  as  suf- 
ficient magnetic  flux  has  been  produced  in 
the  magnetic  circuit,  to  move,  by  magnetic 
attraction,  a  soft  iron  armature  A,  sup- 
ported at  the  extremities  of  the  springy, 
the  spring  is  forced  to  leave  the  contact  c. 
and  thus  open  the  circuit.  As  soon  as  the 
circuit  opens,  the  current  strength  would 
immediately  fall  to  zero,  but  for  the  fact 
that  the  magnetic  flux  in  the  circuit,  being 


ELECTRO-THER^EUTlJ  3.V/  I    L  flMPfy     Q  f 

unsupported  by  M.  M/SEfo»M?id]y  disap-         ^  ^ 
pears,  so  that  the  primary  l8^pg4^£8»pidly; XN^-^ 
emptied  of  flux  and  become  the  seat  of  an   7 
E.  M.  F.  tending  to  prolong  the  current. 
This  E.  M.  F.  is  comparatively  powerful, 
owing  to  the  rapid  rate  at  which  the  flux 
is  emptied;  in  fact,  it  may  be  sufficiently 
great  to  cause  a  spark  to  form  between  the 
spring  and  the  contact  c.     In  this  manner 
both  the  making  and  the  breaking  of  the 
primary  circuit  produce  E.  M.  Fs.  in  the  cir- 
cuit ;  that  on  making,  tending  to  oppose  the 
establishment  of  the  current,  and  that  on 
breaking,  tending  to  oppose  its  cessation. 

On  the  closing  of  the  primary  circuit, 
the  time  occupied  in  producing  the  full 
flux  is  comparatively  great ;  that  is  to 
say,  it  may  amount  to,  perhaps,  the  one 
hundredth  of  a  second ;  the  loops  do  not, 
therefore,  fill  so  rapidly ;  but  when  the  cir- 


256  ELECTRICITY   IN 

cuit  is  opened  at  the  contact  c,  the  flux  is 
necessarily  withdrawn  with  great  rapidity, 
and  the  E.  M.  F.  induced  at  breaking, 
owing  to  this  greater  rate,  is  much  in 
excess  of  the  E.  M.  F.  induced  at  making. 
The  value  of  the  E.  M.  F.  on  breaking  will, 
therefore,  be  increased  by  any  cause  which 
will  tend  to  diminish  the  time  required  for 
the  complete  cessation  of  the  primary  cur- 
rent. The  spark  which  bridges  the  space 
between  the  contact  point  and  the  spring 
has,  therefore,  the  effect  of  prolonging  the 
current,  since  it  provides  a  path,  of  heated 
air,  through  which  the  current  may  flow, 
even  when  the  contact  is  broken.  Conse- 
quently, any  device  which  will  stop  the 
spark  will  result  in  the  production  of  a 
higher  E.  M.  F.  of  self-induction  in  the 
primary  coil.  This  is  sometimes  effected 
by  introducing  a  condenser  into  the  pri- 
mary circuit,  with  its  terminals  connected 


ELECTRO-THERAPEUTICS.  257 

in  shunt  to  the  contact.  As  soon  as  the 
contact  is  broken,  the  current  instead  of 
following  through  the  air  in  a  spark,  rushes 
into  the  condenser  and  charges  it.  As 
soon  as  the  condenser  is  charged,  the  cur- 
rent ceases  very  suddenly,  and  is  then 
unable  to  jump  across  the  interval  of  air 
which  has  become  interposed  between  the 
contact  point  and  the  spring.  The  result 
is,  therefore,  that  the  current  in  the  pri- 
mary coil,  being  very  suddenly  arrested  in 
the  condenser,  the  rate  at  which  the  loops 
are  emptied  of  flux  is  greatly  increased, 
and  the  E.  M.  F.  induced  in  the  primary 
circuit  is  correspondingly  increased. 

So  far  we  have  only  considered  the  pri- 
mary coil  as  though  the  secondary  coil 
were  entirely  removed  from  it.  We  may 
now  consider  the  effects  that  are  produced 
in  the  superposed  secondary  coil.     If  the 


258  ELECTRICITY   IN 

terminals  of  the  secondary  coil  be  opened 
so  that  its  external  resistance  is  practically 
infinite,  it  can,  therefore,  send  no  current. 
The  E.  M.  F.  induced  in  the  secondary 
coil  is  the  exact  counterpart  of  that  which 
is  induced  in  the  primary  coil,  except, 
that  having  more  turns,  the  secondary 
E.  M.  F.  is  correspondingly  greater.  This 
is  on  the  assumption  that  all  of  the  mag- 
netic flux  threading  through  the  primary 
coil,  also  threads  through  the  secondary. 
On  closing  the  primary  circuit,  the  en- 
trance of  magnetic  flux  through  the  pri- 
mary and  secondary  loops  together  causes 
E.  M.  Fs.  to  be  induced  in  both  coils. 
This  E.  M.  F.  is  a  C.  E.  M.  F.  in  the  pri- 
mary circuit,  since  it  acts  against  the 
E.  M.  F.  impressed  upon  it,  but  it  is  the 
only  E.  M.  F.  which  appears  in  the 
secondary  circuit.  If,  for  example,  the 
primary  coil  consists  of  100  turns  of  wire, 


ELECTRO-THERAPEUTICS. 


259 


and  has  induced  in  it  a  C.  E.  M.  F.  com- 
mencing at,  say  one  volt,  as  shown  in  Fig. 
97,  this  C.  E.  M.  F.  dying  away,  along  the 
curved  line  bed;  then,  if  the  secondary 


— i 


-3- 

-4- 
— 5-J 

Fig.  97. — Diagram  of  Primary  Induced  E.  M.  Fs. 


coil  consists  of  5,000  turns,  the  E.  M.  F. 
induced  in  this  secondary  coil  will  be 
represented  by  the  same  curve  on  a  scale 
50  times  as  great;  that  is  to  say,  com- 
mencing at  50  volts,  instead  of  at  1  volt. 


260  ELECTRICITY   IN 

The  moment  that  the  primary  circuit  is 
broken  at  the  contact  <?,  the  E.  M.  F. 
induced  in  the  primary  circuit,  instead  of 
being  1  volt  reducing  to  almost  zero  in 
the  hundredth  of  a  second,  may  be  5  volts, 

reducing  to  zero  in,  perhaps,   ■  nrtn^n  °f  a 

second,  as  is  represented  in  the  Fig.  97,  by 
the  curve  def.  Similarly  in  the  second- 
ary circuit,  the  E.  M.  F.  induced  will  be 
represented  by  the  same  curve  on  a  scale 
50  times  as  great,  and  will  commence  at 
500  volts,  reducing  rapidly  to  zero.  The 
E.  M.  F.  of  such  a  faradic  coil  is,  therefore, 
obviously  alternating,  but  dissymmetrical  in 
character,  the  wave  at  breaking  being  much 
greater  in  amplitude,  but  correspondingly 
more  brief  than  the  wave  at  making. 

If   the  secondary  coil  be  moved  away 
from  the  primary  coil,  as  in  instruments 


.if  Jb Ticp:  n  C  F  Pj^T  V  r  c 

of  the  Dubois-Raymond  t^ib$9§ne  of  which 

is   represented    in    Fig.    9o^&ffafyjtexi,r,  y$$R* 

secondary  coils  are   shown  with 

mary    partly   inserted    in    one    of    them, 


Fig.  98.— Induction  Coil,  Dubois-Raymond  Type. 

the  E.  M.  F.  of  self-induction,  in  the 
primary  circuit,  will  not  be  influenced, 
but  the  E.  M.  F.  of  mutual  induction 
in  the  secondary  coil,  will  be  reduced, 
because  the  amount  of  flux  linked 
with    the    secondary     turns    is    reduced, 


262  ELECTRICITY   IN 

and  finally,  when  the  coils  are  entirely 
separated,  the  amount  of  flux  from 
the  primary  coil,  which  remains  linked 
with  the  secondary  coil,  is  so  small, 
that  its  filling  and  emptying  produces  an 
inappreciable  induced  secondary  E.  M.  F. 
Consequently,  in  the  use  of  the  Dubois- 
Raymond  type  of  coil,  when  it  is  desired 
to  increase  the  secondary  E.  M.  F.,  the 
primary  coil  is  pushed  further  into 
secondary  coi],  or  vice  versa,  so  that  the 
flux  produced  in  the  magnetic  circuit  may 
link  with  more  and  more  turns  in  the 
secondary  coil.  In  no  case,  can  the  E.  M. 
F.  in  a  secondary  coil  be  other  than 
increased  by  bringing  the  primary  and 
secondary  coils  closer  together,  so  long  at 
least  as  the  secondary  circuit  is  open. 

We  have  hitherto  considered  the  second- 
ary coil  as  being  open,  and  have  seen  that 


ELECTRO-THERAPEUTICS.  263 

in  such  cases,  although  E.  M.  Fs.  are  pro- 
duced in  the  secondary,  its  presence  has  no 
appreciable  effect  upon  the  nature  of  the 
phenomena  that  occur  iii  the  primary  cir- 
cuit. As  soou,  however,  as  the  terminals 
of  the  secondary  coil  are  connected  to  an 
external  circuit,  so  that  a  secondary  cur- 
rent can  be  produced  in  such  circuit,  then 
effects  are  produced,  which  are  superposed 
upon  those  already  existing;  namely, 
secondary  currents  flow.  These  second- 
ary currents  produce  a  M.  M.  F.  in  the 
secondary  coil,  and  a  magnetic  flux  through 
the  secondary  coil,  part  of  which  passes 
through  the  primary  coil.  This  secondary 
flux,  filling  and  emptying  the  secondary 
loops,  sets  up  a  C.  E.  M.  F  of  self-induc- 
tion in  the  secondary  circuit  tending  to 
oppose  both  the  development  and  the 
extinguishment  of  each  current  wave. 
Moreover,  the  flux  from  the  secondary  cir- 


264  ELECTRICITY    IN 

cuit,  passing  through  the  primary  coil, 
induces  in  it  an  E.  M.  F.  by  mutual 
induction,  and  disturbs  the  flow  of  current 
strength  through  the  primary  coil.  It  is 
to  this  disturbance,  and  to  the  flow  of 
primary  current  against  the  mutually 
induced  C.  E.  M.  R,  that  the  energy  is 
obtained  from  the  primary  circuit  which 
appears  in  the  secondary  circuit.  The 
amount  of  reaction  taking  place  from 
the  secondary  back  into  the  primary 
circuit,  depends  upon  the  strength  of 
the  secondary  currents,  which  in  its  turn 
depends  upon  the  resistance  of  the  second- 
ary circuit  as  composed  partly  of  the 
resistance  in  the  secondary  coil,  and 
partly  in  the  exterual  resistance,  and  also 
upon  the  self-induction,  or,  as  it  is  usually 
called,  the  inductance  of  the  secondary  coil 
already  mentioned.  The  greater  the  cur- 
rent   strength    in   the   secondary    circuit, 


ELECTRO-THERAPEUTICS.  265 

the  greater  the  mutually  induced  C.  E.  M. 
F.  in  the  primary  circuit,  and  the  greater 
the  reactive  disturbance  in  magnetic  flux 
and  in  any  current  strength  there  existing. 

Although,  when  the  secondaiy  circuit  is 
open,  its  E.  M.  F.  at  the  peak  of  the  wave 
on  breaking,  may  measure  200  or  300 
volts,  yet  on  closing  the  secondary  circuit, 
through  a  comparatively  low  resistance, 
the  E.  M.  F.  at  the  secondary  terminals 
may  only  be  2  or  3  volts.  It  is  evident, 
that  if  the  secondaiy  terminals  be  short 
circuited  by  a  stout  piece  of  wire  having 
negligible  resistance,  the  E.  M.  F.  at 
secondary  terminals  would  be  0  volts,  but 
even  when  the  secondary  external  resist- 
ance is  say,  1,000  ohms,  the  E.  M.  F. 
available  at  secondary  terminals  is  always 
very  small  in  the  medical  induction  coil. 
This  is  both  for  the  reason  that,  owing  to 


ELECTRICITY   IN 

the  reaction  of  the  secondary  M.  M.  F.  of 
the  primary  circuit,  the  E.  M.  F.  in  the 
secondary  coil  is  reduced,  and  because 
such  E.  M.  F.  as  remains,  so  soon  as  it 
tends  to  produce  such  a  current  in  the  ex- 
ternal circuit,  as  would  flow  in  accordance 
with  Ohm's  law,  is  met  by  a  powerful 
C.  E.  M.  F.  due  to  its  self-induction. 
This  C.  E.  M.  F.  is  most  powerful  when 
the  wave  of  induced  E.  M.  F.  is  most 
abrupt,  since,  at  such  times,  the  inrush  of 
magnetic  flux  into  the  secondary  coil  from 
its  own  rising  current  is  greatest.  The  re- 
sult is  that  the  drop  of  pressure  in  the  coil 
is  always  very  great,  owing  to  its  large 
resistance  and  large  inductance ;  i.  e.y  its 
large  number  of  turns  and  capability  of 
producing  C.  E.  M.  F.  by  self-induc- 
tion. The  current  which  can  flow  in 
the  secondary  circuit  is,  therefore,  not 
only    comparatively    feeble,   but    is    also 


ELECTRO-THERAPEUTICS.  26^ 

much  less  abrupt  in  wave  character  than 
the  induced  E.  M.  F.  would  lead  one  to 
expect. 

The  core  employed  in  the  medical  induc- 
tion coil  almost  invariably  consists  of  a 
bundle  of  soft  iron  wires.  A  solid  rod  of 
soft  iron  cannot  be  efficiently  employed, 
because,  under  the  influence  of  the  rapidly 
oscillating  magnetic  flux  through  such  a 
rod,  electric  currents  would  be  produced 
around  it  in  such  a  manner  as  to  oppose 
the  development  of  the  magnetization ; 
that  is  to  say,  the  external  shells  of  the 
rod  would  act  as  closed  secoudary  coils, 
tending  to  check  and  react  upon  the  pri- 
mary current  and  wasteful  ly  expend 
energy.  By  employing  a  bundle  of  fine, 
soft  iron  wires,  all  these  induced  eddy 
currents  are  reduced  to  negligibly  small 
strengths 


268  ELECTRICITY   IN 

If  the  core,  instead  of  being  limited  to 
the  interior  of  the  coil,  be  brought  around, 
so  that  its  extremities  shall  meet,  and  the 
core  thus  entirely  surround  the  coil,  the 
magnetic  flux,  produced  in  the  magnetic 
circuit  by  a  given  primary  M.  M.  F.,  will 
be  much  greater.  It  might,  therefore,  at 
first  sight  be  supposed  that  such  an 
arrangement  would  increase  the  induced 
secondary  E.  M.  F.;  but  while  the  flux 
would  certainly  be  increased,  the  residual 
magnetism  in  the  ring  of  iron  thus  formed 
would  be  so  considerable,  that  the  rate  at 
which  flux  could  be  caused  to  oscillate  in 
the  ring  would  be  reduced  in  a  greater 
proportion  than  the  total  flux  could  be 
increased,  so  that  the  employment  of  such 
a  ring  would  be  detrimental.  If  the  pri- 
mary current,  instead  of  being  pulsatory 
or  unidirectional,  were  alternating,  its  in- 
fluence in  eliminating  the  residual  magnet- 


ELECTRO-THERAPEUTICS.  269 

tism,  at  each  reversal,  would  be  much 
greater,  and  the  benefit  of  a  completely 
ferric  circuit  would  be  proportionate. 

The  rate  of  vibration  of  the  spring  con- 
tact has  an  important  bearing  upon  the 
action  of  the  apparatus.  The  frequency 
of  the  primary  pulsating  current  depends 
upon  the  natural  frequency  of  vibration 
of  the  spring,  which  in  turn  depends  upon 
its  length,  breadth,  thickness,  elasticity  and 
the  weight  of  the  armature  it  supports 
at  its' free  end.  The  lowest  tone,  which 
the  spring  vibrator  will  emit,  corresponds 
to  the  slowest  natural  rate  of  vibration 
at  which  the  spring  will  vibrate  as  a 
whole.  By  causing  the  contact  to  ap- 
proach the  core,  so  as  to  reduce  the  range 
of  vibration,  the  spring  is  aided  in  produc- 
ing overtones  /  i.  e.,  vibrations  set  up  from 
the  contact  point  as  a  node,  and  in  some 


270  ELECTRICITY   IN 

cases  the  vibration  of  the  spring  is  com- 
posite, being  partly  performed  as  a  Whole, 
and  partly  in  segments,  having  the  con- 
tact point  as  node. 

The  effect  of  increasing  the  frequency  of 
vibration  in  the  first  instance  is  to  increase 
the  E.  M.  F.  induced  in  the  primary  and 
secondary  coils  by  self  and  mutual  induc- 
tion owing  to  the  greater  rapidity  with 
which  the  flux  is  compelled  to  enter  and 
leave  the  coils.  On  the  other  hand,  when 
the  rapidity  of  vibration  exceeds  a  certain 
value,  depending  upon  all  the  electric  con- 
ditions of  the  circuit,  the  time  in  each 
pulsation,  during  which  contact  is  main- 
tained at  the  vibrating  spring,  is  so  short 
that  the  entering  primary  current  is  greatly 
reduced  in  strength,  since  it  lias  not  time 
to  overcome  the  C.  E.  M.  F.  of  self  induc- 
tion.    On   this   account  the   primary  cur- 


ELECTRO-THERAPEUTICS.  271 

rent  waves  are  reduced  in  amplitude,  and 
the  M.  M.  F.  is  correspondingly  reduced, 
thereby  restricting  the  development  of 
flux.  When,  therefore,  this  particular 
speed  of  vibration  has  been  obtained, 
the  gain  in  secondary  E.  M.  F.,  by 
reason  of  the  higher  frequency,  is  offset 
and  neutralized  by  the  loss  of  total  flux 
produced  in  the  magnetic  circuit.  More- 
over, the  greater  the  rapidity  with  which 
the  secondary  waves  of  E.  M.  F.  are  in- 
duced, the  more  rapid  will  be  the  waves 
of  M.  M.  F.  and  flux  in  the  secondary  coil, 
and  the  greater  the  secondary  C.  E.  M.  F. 
of  self-induction  tending  to  oppose  the 
development  of  abrupt  changes  in  the 
secondary  current  strengths.  Although 
the  effect  of  these  separate  influences  de- 
pends upon  the  particular  proportions  of 
each  induction  coil,  yet,  broadly  speaking, 
the  effect  of   increasing  the  frequency  of 


272  ELECTRICITY   IN 

vibration  in  the  contact  spring  may  be 
expressed  as  follows.  In  all  cases  the  fre- 
quency in  the  secondary  circuit  increases 
in  conformity  with  the  increase  in  the 
primary.  Up  to  a  certain  frequency  both 
the  E.  M.  F.  and  the  current  strength  in 
the  secondary  circuit  are  increased ;  beyond 
this  frequency,  the  E.  M.  F.  in  the  second- 
ary circuit  is  not  increased,  but  the  wave 
type  of  secondary  current  becomes  modi- 
fied. The  more  rapid  the  vibration,  the 
smoother  and  less  abrupt  the  current 
waves  produced,  particularly,  when  long 
fine-wire  secondary  coils  are  employed, 
owing  to  the  powerful  C.  E.  M.  F.  set  up 
by  rapid  fluctuations,  and  the  throttling 
effect  thus  produced  upon  all  abrupt  varia- 
tions of  current  strength. 

In  order,  therefore,  to  produce  in  the 
secondary  circuit  abrupt  waves  of  the  type 


ELECTR0-TH1 


diagrammatically  slio^n^p    Fig.  97,   the 
secondary  circuit  snoukiN^a?^  ^ompara- ■  Oo^^ 
tively   small    number   of    t  rrmn^nf-  yn ytr^*^ 
placed  as  close  as  possible  to  the  primary 
coil,  so  that  the  throttling  effect  of  self-in- 
duction in  its  own  turns  may  be  avoided, 


Fig.  99.— Type  of  Secondary  Induced  E.  M.   P.   at 
High  Frequencies  Under  Load. 

and  the  frequency  of  vibration  compara- 
tively small.  On  the  other  hand,  in  order 
to  produce  in  the  secondary  circuit, 
smooth,  less  abrupt  current  waves  such  as 
those  shown  in  Fig.  99,  long,  fine-wire 
coils,  with  considerable  inductance,  should 
be  employed.  Moreover,  it  is  not  necessary 
that  all  the  turns  should  be  linked  with 


274  ELECTRICITY   Itt 

the  primary  coil ;  that  is  to  say,  the  second- 
ary E.  M.  F.,  produced  by  mutual  induc- 
tion, can  be  induced  in  part  of  the  second- 
ary coil.  The  remainder  of  the  second- 
ary coil,  being  unacted  on,  will  serve  to 
choke  or  throttle  the  sudden  variations  in 
the  secondary  current  by  its  inductance. 
This  effect  will  be  enhanced  by  making 
the  frequency  conveniently  great.  It  is  to 
be  remembered,  however,  that  in  no  case 
can  either  the  E.  M.  F.  wave,  or  the 
current  wave,  in  the  secondary  circuit  be 
made  symmetrical,  the  wave  on  breaking 
being  always  steeper  than  that  on  making. 

The  usual  method  of  altering  the  fre- 
quency in  the  primary  circuit,  consists  in 
advancing  the  spring  contact,  so  as  to  in- 
crease the  tension  of  the  vibrating  spring, 
and  reduce  its  range  of  vibration.  The 
range  of  frequency  obtained  in  this  way  is 


ELECTRO-THERAPEUTICS. 


275 


comparatively  small,  as  it  is  rarely  that  the 
frequency  can  be  doubled  in  this  manner. 
A  device  sometimes  employed  in  connection 


&AITZBX 

Fig  100.— Adjustable  Vibrator  for  Faradic  Coils. 

with  larger  induction  coils  is  shown  in 
Fig.  100,  where  the  screw  S,  and  the  nut  JV, 
serve  to  tighten  the  vibrating  spring  and 
thus  vary  its  tension.  The  frequency  ob- 
tained from  an  ordinary  spring  vibrator  is 


276  ELECTRICITY   IN 

from  150  to  300-  ;  i.  e.,  150  to  300  com- 
plete periods  per  second.  For  higher  fre- 
quencies a  ribbon  vibrato?*  is  sometimes 
employed,  such  as  shown  in  Fig.   101. 


Fig.  101.— Induction  Coil  with  Ribbon  Vibrator. 

In  a  ribbon  vibrator  the  contact  screw 
(7,  presses  upon  a  horizontal  steel  ribbon, 
at  a  point  which  is  about  one  quarter  of 
the  length  of  the  ribbon  from  the  fixed 


ELECTRO-THERAPEUTICS.  277 

support  on  the  left.  The  ribbon  can  be 
tightened  by  the  thumbscrew  on  the  right 
hand  pillar  or  support.  By  varying  the 
tension  on  the  ribbon,  a  considerable 
range  of  frequency  of  vibration  can  be  ob- 
tained. The  induced  secondary  currents 
obtained  from  such  an  apparatus,  with  a 
long,  thin- wire  coil,  have  considerable  fre- 
quency, small  strength,  and  comparatively 
smooth,  wave  character.  For  abrupt, 
powerful,  secondary  currents,  a  slow  speed 
vibrator  is  represented  at  V,  in  the  same 
figure.  It  consists  of  a  vertical  electro- 
magnet, which  attracts  an  armature  of 
soft  iron,  attached  to  a  horizontal  steel 
spring,  and  is  loaded  with  a  spherical 
weight,  capable  of  being  clamped  at  vary- 
ing distances  along  the  rod  attached  to  the 
moving  system.  By  clamping  the  weight 
near  the  free  extremity  of  the  rod,  the 
slowest  vibrations  are  obtained ;  while  by 


27a 


ELECTRICITY   IN 


clamping  it  near  to  the  armature,  the 
speed  of  vibration  is  increased.  The 
secondary  coil  employed  with  the  slow 
vibrator    is    usually   of   fewer   turns  and 


^■IeS.        ^~?H*f£3fJ? 

f'^wr~^^^^A  H* 

1 

1 

Kki^    g^ 

rKJh3^ 

T      ^r  T?__* 

T  7  *  «J5 *?  s       ^  ~  ^            " '  ~  *     |S, 

• 

Fig.  102.— Induction  Coil  with   Rapid  Interrupter. 


coarser  wire,  offering  a  smaller  resistance 
and  a  considerably  smaller  inductance. 

Fig.  102,  represents  a  type  of  medical 
induction  coil  in  which  the  frequency  is 
varied  by  means  of  a  small  electromagnetic 
motor    MM.      This    motor    drives    three 


ELECTRO-THERA 

wheels  1,  2  and  3,  upon 
which  are  a  series  of 
which  are  pressed  upon  by  spring  col 
By  the  insertion  of  one  of  these  wheels  in 
the  primary  circuit  of  the  induction  coil, 
the  frequency  of  pulsation  of  the  primary 
current  can  be  varied  within  wide  limits, 
and  a  high  frequency  attained.  The 
secondary  coil  C,  can  be  moved  toward  or 
from  the  primary  coil  within  it,  by  turn- 
ing the  screw  S. 

Another  method  of  varying  the  induced 
E.  M.  F.  in  a  secondary  coil,  without  vary- 
ing the  frequency,  is  by  the  use  of  a 
metallic  tube  inserted  between  the  core 
and  the  primary  coil,  between  the  primary 
and  secondary  coils,  or  over  the  secondary 
coil.  The  second  method  is  represented  in 
Fig.  103,  where  the  handle  Hy  is  attached 
to  one  extremity  of  a  brass  tube,  inserted 


280  ELECTRICITY    IN 

between  the  primary  and  secondary  coils 
of  the  medical  induction  coil,  which  in  this 


Fig.  103. — Induction  Coil  with  Dry  Cell  and 
Internal  Damping  Tube. 

instance  is  supplied  by  a  dry  voltaic  cell, 
situated  in  the  base  of  the  apparatus.  An 
example  of  the  third  method  is  represented 
in  Fig.  104,  where  the  tube  T,  is  advanced 


ELECTRO-THERAPEUTICS. 


281 


over    the    surface  of  the    secondary   coil. 
The  action  of  the  tube  is  in  all  cases  the 


Fig.  104.— Induction  Coil  with  External  Damping 
Tube. 

same.     It  forms,  in  reality,  an  independent 
secondary  coil,  having  a  very  low  resist- 


282  ELECTRICITY   IN 

ance,  consisting  as  it  does  of  but  a  single 
turn  of  conductor.  When  the  magnetic 
flux  from  the  primary  coil  and  core,  fills 
and  empties  this  tube,  it  sets  up  around 
it,  comparatively  powerful  induced  current 
strengths,  owing  to  its  low  resistance. 
Although  the  induced  E.  M.  F.  may  be  but 
feeble,  yet  with  the  presence  of  a  single 
turn  the  effect  of  this  comparatively 
powerful  secondary  current  is  to  produce 
an  M.  M.  F.  and  magnetic  flux  with  such 
reactive  power  upon  the  primary  circuit, 
that  the  resultant  magnetic  flux,  linked 
with  the  secondary  coil,  is  very  greatly 
enfeebled.  The  further  the  tube  is  pushed 
into  or  over  the  coil,  the  greater  will  be 
its  reactive  influence  upon  the  primary  cir- 
cuit, and  the  smaller  will  be  the  induced 
E.  M.  F.  in  the  secondary  coil. 

The  Dubois-Eaymond  method  of  vary- 


ELECTRO-THERAPEUTICS.  283 

ing  the  secondary  E.  M.  F.  consists,  there- 
fore, of  varying  the  mutual  inductive 
power  of  the  primary  and  secoodary  coils, 
by  varying  their  mean  distance  from  each 
other.  The  shield  method  consists  in 
leaving  the  primary  and  secondary  coils 
at  a  fixed  distance,  but  so  distributing 
the  magnetic  flux  through  the  coils, 
under  the  influence  of  powerful  shield 
secondary  currents,  that  the  resultant  mag- 
netic flux  through  the  secondary  circuit  is 
reduced. 

Notwithstanding  the  great  variety  of 
medical  induction  coils,  and  their  seeming 
differences  of  construction,  electrically  they 
invariably  consist  of  a  primary  and  second- 
ary coil  in  mutual  inductive  relation  to 
each  other,  and  an  interrupter,  to  vary  the 
frequency  of  the  primary  current.  The 
primary  coil  may  possess  a  greater  or  less 


284  ELECTRICITY   IN 

resistance  and  inductance,  and  the  two 
coils  may  be  so  placed  as  to  have  a  greater 
or  less  mutual  inductance.  The  number 
of  turns  in  the  primary  and  secondary  coils, 
the  number  of  cells  to  be  employed  to 
operate  the  instrument,  and  the  manner 
in  which  the  mutual  inductive  influence 
between  the  coils  is  varied,  are  merely, 
from  an  electric  standpoint,  incidental 
to  the  main  purpose  for  which  the  coil 
is  intended;  namely,  to  produce  at  the 
secondary  terminals  an  E.  M.  F.  of  a  given 
frequency,  value,  and  wave  type,  when 
the  external  resistance  is  assigned.  That 
is  to  say,  if  the  resistance  of  the  external 
circuit  comprising  the  body  of  a  patient, 
and  the  electrodes  connected  therewith, 
amounts,  to  say  2,000  ohms,  then  the  func- 
tion of  the  apparatus  is  to  produce  at  its 
secondary  terminals  a  certain  effective  volt- 
age, such  as  would  be  indicated  by  a  suit- 


ELECTRO-THERAPEUTICS.  285 

able  voltmeter,  a  certain  frequency,  and  a 
certain  wave  character  of  E.  M.  F.  If  the 
effective  pressure  under  these  circumstances 
at  the  secondary  terminals  be,  say  6  volts, 
then  the  effective  current  through  the 
external  circuit,  provided  that  the  induct- 
ance of  the  external  circuit  is  small ;  i.  e., 
that  there  are  no  coils  of  many  turns  of 
wire  in  the  circuit,  will  by  Ohm's  law,  be 

%m  =  im  = 3  miiiiamPeres- 

In  order  to  compare  the  relative  electric 
effectiveness  of  different  medical  induc- 
tion coils,  it  is  only  necessary  to  measure 
the  effective  E.  M.  F.  maintained  at  the 
terminals  of  the  secondary,  when  con- 
nected through  different  external  induc- 
tionless  resistances,  the  frequency  of  the 
interrupted  and  the  wave  type  of  E.  M.  F. 
being   at   the   same   time    observed.     For 


286  ELECTRICITY  IN 

some  apparatus  a  high  frequency,  and  a 
smooth  type  of  wave  are  the  desiderata, 
while  for  others,  a  low  frequency,  and  a 
rough  wave  type  are  desired.  The  coil 
which  will  supply  the  requisite  number  of 
volts  at  its  secondaiy  terminals,  under  all 
variations  of  load,  and  will,  at  the  same 
time,  supply  these  frequencies  and  wave 
type  with  the  maximum  economy,  sim- 
plicity, durability  and  convenience,  will 
be  the  most  effective  coil,  from  an  electric 
standpoint,  and  no  structural  variations  in 
an  induction  coil,  whether  obtained  by 
altering  its  magnetic  circuit,  its  winding, 
or  its  various  parts,  can  do  anything 
except  to  alter  the  frequency,  the  wave 
type  and  the  magnitude  of  the  E.  M.  F.  at 
its  secondary  terminals. 

It   might    be    supposed    that    when   a 
medical  induction  coil,  of  the  Dubois- Ray- 


ELECTRO-THERAPEUTICS.  287 

mond  type,  has  its  secondary  coil  as  far 
over  the  primary  coil  as  it  will  go,  that 
the  current  strength  in  the  secondary  cir- 
cuit will  be  a  maximum.  This,  however, 
is  not  always  the  case,  since  the  M.  M.  F. 
of  the  current  in  the  secondaiy  coil  may 
so  greatly  react  upon  the  primary  flux  and 
current,  as  to  cause  the  secondary  to  act 
like  a  metallic  shield.  In  other  words, 
the  secondary  coil  may  overload  the 
primary.  For  this  reason,  it  is  possible 
that  the  maximum  current  strength  in  the 
secondary  circuit  may  be  obtained  when 
the  coil  is  only  partly  covering  the  pri- 
mary, say  one  half  or  three  quarters. 

In  a  continuous-current  circuit,  since  the 
E.  M.  F.  is  always  acting,  there  is  no  diffi- 
culty in  determining  its  value ;  but  since,  in 
an  alternating-current  circuit,  the  value  of 
the  E.  M.  F.  reaches  a  maximum  twice  in 


288  ELECTRICITY   IN 

every  cycle,  varying  on  each  side  of  zero,  it 
is  more  difficult  to  define  what  is  the  value 
of  the  E.  M.  F.  The  E.  M.  F.  is  always 
defined  as  the  effective  thermal  F.  M.  F.  or 
simply  the  effective  F.  M.  F.  That  is  to 
say  the  value  of  an  alternating  or  pulsat- 
ing E.  M.  F.  is  taken  as  being  equal  to 
that  of  the  continuous  E.  M.  F.,  which  is 
capable  of  producing  the  same  heating  ef- 
fect in  an  inductanceless  resistance.  Simi- 
larly, when  the  electric  current  pulsates 
or  varies,  its  effective  thermal  value,  com- 
monly called  its  effective  current  strength, 
is  equal  to  that  strength  of  continuous  cur- 
rent which  would  produce  in  a  given  re- 
sistance the  same  heating  effect.  If,  there- 
fore, the  effective  E.  M.  F.  at  the  terminals 
of  a  medical  induction  coil  be  expressed  or 
measured  as  five  volts  effective,  we  mean 
that  the  E.  M.  F.  it  produces,  although 
varying   between,   perhaps,   20    volts    and 


^ 


j^T&  OlESf IfljSj 


electro-tiim^pelPI^^.  p  r  ri8f  f  L 

zero,  produces  in  a  sr»$£  resistance,  the 
same  amount  of  heat  l^fe^uvr.^p,^ 
a  minute  or  five  minutes, 
of  continuous-current  pressure.  Conse- 
quently, the  statement  of  the  effective 
current  strength  or  effective  E.  M.  F.  at 
the  terminals  of  an  induction  coil  does  not 
express  the  range  of  current  or  of  E.  M.  F. 
in  each  wave,  unless  the  shape  of  the 
wave  be  known. 

The  connections  of  the  medical  induc- 
tion coil  are  commonly  so  arranged  that 
the  E.  M.  F.  induced  in  either  the  primary 
or  the  secondary  circuit  can  be  externally 
employed.  We  have  seen  that  on  break- 
ing, an  E.  M.  F.  of  self-induction  appears 
in  the  primary  circuit,  as  well  as  an  E.  M. 
F.  of  mutual  induction  in  the  secondary 
circuit,  but  the  E.  M.  F.  of  mutual  induc- 
tion is  practically  always  greater  than  in 


290 


ELECTRICITY   IN 


the  primary  circuit  owing  to  the  greater 
number  of  turns  in  the  secondary  coil.  If 
Plf  P2,  Fig.  105,  represent  the  terminals  of 
the  primary  coil,  and  /Si,  S%  the  terminals  of 


Pl  P3  P2  S,  s, 

Fig.  105. — Connections  op  Medical  Induction  Coil. 


the  secondary  coil,  while  Pi,  is  connected 
to  the  vibrating  spring,  then  it  will  be 
seen  that  when  the  spring  breaks  contact, 
the  induced  E.  M.  F.  in  the  primary  coil 
will  be  developed  between  the  terminals 
P%  and  Pt,  while  the  mutually  induced  E. 


ELECTRO-THERAPEUTICS.  .      291 

M.  F.  will  be  developed  between  $,  $». 
There  is  this  difference,  however,  between 
the  E.  M.  F.  of  the  primary  and  secondary 
circuits,  that  wThereas,  S\  and  $,  will 
supply  an  E.  M.  F.  both  at  making  and  at 
breaking,  in  opposite  directions,  although 
the  E.  M.  F.  at  breaking  is,  as  we  have 
seen,  much  stronger  than  the  E.  M.  F.  at 
making ;  yet,  on  the  other  hand,  between 
P,  and  P3y  the  E.  M.  F.  on  making  is  always 
very  small,  and  must  be  less  than  the  E. 
M.  F.  between  Px  and  P* ;  that  is  to  say, 
it  must  be  less  than  the  E.  M.  F.  of  the 
batteiy.  Under  ordinary  circumstances, 
therefore,  when  the  primary  coil  has  com- 
paratively few  turns  of  coarse  wire,  the 
current  obtainable  between  P2  and  P3,  is 
practically  zero  at  making,  and  rises  sud- 
denly and  abruptly  on  breaking,  or  acts 
exactly  in  the  same  manner  as  a  secondary 
coil  of  the  same  number  of  turns.     Such  a 


292  ELECTRICITY   IN 

type    of   medical    induction    coil  is  repre- 
sented in  Fig.  106. 

In  Fig.  107,  a  common  form  of  connec- 
tion of  the  circuits  of  a  medical  induction 


Fig   106.— Induction  Coil   with  Handle  Electrodes. 

coil  is  shown.  Here  one  end  of  the  sec- 
ondary coil  is  brought  into  connection  with 
one  end  of  the  primary  coil.  By  this  means 
we  obtain  between  P*  and  K,  the  self- 
induced  E.  M.  F.  of  the  primary,  and  be- 


ELECTRO-THERAPEUTICS. 


293 


tween  /Si  and  /&,  the  mutually  induced  E. 
M.  F.  of  the  secondary  coil.  Between  P* 
and  &,  the  total  E.  M.  F.  in  both  coils  is 
obtained.  The  effect  in  fact  is  to  add  to 
the  secondary  coil  another  layer  which  is 


Fig.  107. — Inter-connection  op  Primary  and  Second- 
ary Windings  in  Medical  Induction  Coil, 


actually  serving,  before  breaking  contact, 
as  a  primary.  At  making,  however,  the 
effect  due  to  the  secondary  coil  is  greater 
than  the  effect  due  to  the  primary  coil. 
It  is  to  be   remembered    that   when   any 


294  ELECTRICITY   IN 

combination  takes  place,  by  the  superposi- 
tion of  a  secondary  current  upon  a  primary 
current,  or  of  either  upon  a  continuous 
current  in  a  circuit,  the  result  will  be  the 
sum  of  all  the  influences  acting  independ- 
ently, and  will  consist  electrically,  of  a 
pulsating  or  alternating  wave  of  current, 
having  a  wave  type  which  may  be  modi- 
fied by  the  simultaneous  influence  of  the 
various  components. 

Fig.  108,  shows  a  form  of  apparatus  con- 
sisting of  a  medical  induction  coil  and  of 
a  battery  of  silver  chloride  cells.  The  in- 
duction coil  can  be  operated  from  a  few  of 
the  cells  in  this  battery  so  as  to  supply 
dissymmetrical  alternating  currents  of,  per- 
haps, 200~,  while  the  battery,  which  will 
probably  have  an  E.  M.  F.  of  50  volts, 
will  be  capable  of  supplying  a  continuous 
current. 


ELECTRO-THERAPEUTICS. 


295 


We  have  already  seen  that  both  the  E. 
M.  F.  and  the  current  strength  at  making, 


Fig.  108. — Induction  Coil  with  Battery  of  Chloride 
of  Silver  Cells. 

are  smaller  than  the  E.  M.  F.  and  current 
strength  on  breaking.  It  is  to  be  remem- 
bered, however,  that  in  all  cases,  the  total 
quantity  of  electricity  which  passes  through 


296  ELECTRICITY   IN 

the  circuit  is  the  same  in  each  alternating- 
current  wave.  In  the  secondary  circuit  of 
an  induction  coil,  no  matter  how  great  may- 
be the  dissymmetry  of  wave  type,  that  is 
to  say,  no  matter  if  the  current  strength  is 
much  greater  on  making  than  on  break- 
ing, it  will  be  correspondingly  briefer  in 
duration.  In,  for  example,  Fig.  97,  which 
represents  dissymmetrical  alternating  cur- 
rents produced  in  the  secondary  circuit  of 
an  induction  coil,  the  wave  ((bed,  in  one 
direction,  or  above  the  line,  being  produced 
at  making  contact,  and  def,  the  greater 
wave,  being  produced  at  breaking  contact, 
then  the  area  of  the  wave  abed  as  rep- 
resented graphically,  will  be  equal  to  the 
area  def,  under  all  circumstances.  This 
area  also  represents  the  total  quantity  of 
electricity,  measured  in  coulombs,  which 
will  pass  through  the  circuit.  Conse- 
quently,   all    physiological   effects,    which 


ELECTRO-THERAPEUTICS.  297 

depend  upon  current  strength,  will  be  dis- 
symmetrical or  polar  in  the  secondary  cir- 
cuit ;  that  is  to  say,  the  effect  produced  at 
making  will  be  different  from  the  effect 
produced  on  breaking  and,  consequently, 
the  effects  at  one  pole  will  be  different 
from  those  at  the  other;  but  all  physio- 
logical effects,  which  depend  only  on  the 
quantity  of  electricity,  will  be  the  same 
both  on  making  and  on  breaking.  Thus, 
it  is  well  known  that  muscular  excitability 
is  a  function  of  the  current  strength,  so 
that  the  muscular  excitation  produced  by 
the  making  and  breaking  currents  will  be 
different.  On  the  other  hand,  all  the  elec- 
trolytic effects,  which  are  dependent  upon 
the  quantity  of  electricity  which  passes, 
will  be  alternately  produced  in  equal 
amounts  at  each  wave.  Consequently, 
either  electrolysis  will  not  appear  at 
all,  or  the   products   of    electrolysis   will 


298  ELECTRICITY. 

appear   in   equal   quantities  at  each  elec- 
trode. 

To  sum  up,  the  medical  induction  coil 
gives  discharges  possessing  the  following 
characteristics : 

It  produces  dissymmetrical  alternating- 
current  waves  of  fairly  high  frequency, 
but  intermittent,  the  waves  being  sepa- 
rated by  intervals  of  no  current.  The 
waves  on  breaking  are  steep  and  abrupt, 
except  in  the  case  of  very  long,  fine  wire 
coils,  with  large  self  inductance,  and  chok- 
ing effect.  The  E.  M.  F.  induced  in  the 
secondary  coil  may  be  hundreds  of  volts, 
but  the  E.  M.  F.  available  at  the  termi- 
nals, under  any  ordinary  load,  will  be  not 
usually  more  than  15  volts  effective. 


CHAPTER  XL 


A  foem  of  electric  source,  occasionally 
used  in  electro-therapeutics,  is  to  be  found 
in  the  dynamo-electric  generator.  Of  the 
sources  already  discussed,  the  voltaic  bat- 
tery produces  a  continuous  E.  M.  F. ;  the 
influence  machine,  a  pulsatory  E.  M.  F. ; 
and  the  induction  coil  an  alternating  E.  M. 
F.  Dynamos,  on  the  contrary,  can  be  em- 
ployed to  produce  either  continuous  or 
alternating  E.  M.  Fs.,  according  to  their 
construction.  Those  producing  continuous 
E.  M.  Fs.  are  generally  termed  simply 
dynamos,  or  generators;  while^those  which 
produce  alternating  E.  M.  Fs.,  are  usually 
called  alternators. 


300  ELECTRICITY   IN 

The  fundamental  principle  employed  in 
all  dynamos  and  alternators,  is  the  induc- 
tion of  E.  M.  F.  by  filling  and  emptying 
conducting  loops  with  magnetic  flux. 
Dynamos  and  alternators  generally  consist 
of  a  fixed  and  a  rotary  part,  by  the  mutual 
action  of  which  conducting  loops  become 
filled  and  emptied  with  magnetic  flux 
during  rotation.  The  E.  M.  Fs.  so  gene- 
rated will  be  alternating,  unless  a  com- 
mutator be  employed  to  render  them  con- 
tinuously directed  in  the  external  circuit. 
Any  dynamo,  therefore,  which  employs  no 
commutator  is  necessarily  an  alternator. 
This  has  been  already  illustrated  in  the  case 
of  the  hand  magneto-generator  described  in 
Chapter  VIII. 

The  E.  M.  F.  obtained  from  a  continu- 
ous-current generator  is  always  slightly 
pulsating,  as  represented  in  Fig.  40.     The 


ELECTRO-THERAPEUTICS.  301 

departure  from  a  continuous  E.  M.  F.,  such 
as  is  produced  by  a  voltaic  battery,  is  less 
marked  when  the  number  of  commutator 
bars  is  great.  When,  however,  few  bars 
are  employed,  the  pulsation  may  be  more 
evident,  as  shown  in  Fig.  41.  If  it  were 
possible  to  employ  an  indefinitely  great 
number  of  commutator  bars,  the  E.  M.  F. 
would  be  unvarying.  This  deviation  from 
continuity  occurs  at  each  passage  of  a  com- 
mutator bar  or  segment  beneath  the  brush. 

A  dynamo,  whether  continuous  or  al- 
ternating, usually  employs  electromagnets 
to  supply  the  flux  with  which  its  loops  of 
wire  are  filled  and  emptied.  In  the  case 
of  the  magneto-generator  illustrated  in  Fig. 
93,  a  permanent  magnet  is  employed  for 
this  purpose.  Electromagnets  require  to 
be  supplied  with  a  continuous  current  for 
their  excitation.     This  exciting  current  is 


302  ELECTRICITY  IN 

either  supplied  from  the  armature  of  the 
dynamo  itself,  or  from  some  separate 
source.  In  the  former  case  the  machine  is 
said  to  be  self -excited,  and  in  the  latter, 
separately-excited.  When  the  external  cir- 
cuit of  a  continuous  or  alternating-current 
generator  is  opened,  the  machine  requires 
no  more  power  for  its  operation  than  that 
necessary  to  overcome  its  frictions,  to- 
gether with  the  small  amount  of  electrical 
power  supplied  for  the  excitation  of  the 
field  magnets.  When,  however,  the  ma- 
chine furnishes  a  current  to  an  external 
circuit,  the  power  which  has  to  be  supplied 
to  drive  the  dynamo  is  increased  by  the 
amount  of  electric  power  in  the  external 
circuit.  If,  for  example,  a  machine  ex- 
pends 50  watts,  or  volts-amperes,  in  its 
external  circuit,  the  power  applied  to 
drive  it  is  increased  by  something  more 
than  50  watts,  since  some  loss  of  power 


ELECTRO-THERAPEUTICS.  303 

occurs  in  the  machine  in  order  to  furnish 
the  external  work. 

Continuous-current  generators  are  not 
very  extensively  used  in  electro-thera- 
peutics, since  other  sources  of  continuous 
E.  M.  F.  are  available.  When  generators 
are  used,  they  are  generally  driven  by  elec- 
tric motors,  taking  their  power  from  neigh- 
boring electric  circuits.  For  example,  if  it 
be  required  to  obtain  a  low  continuous 
pressure,  of  say  5  volts,  in  order  to  oper- 
ate an  electric  cautery,  and  an  electric 
circuit  of  higher  pressure,  be  available  in 
the  neighborhood,  say  of  110  or  220  volts 
pressure,  as  commonly  used  in  electric 
lighting,  a  small  motor,  operated  from  the 
lighting  mains,  may  be  employed  to  drive 
a  dynamo  constructed  to  give  the  neces- 
sary E.  M.  F.  and  current  required  for 
the  cautery  knife. 


304  ELECTRICITY   IN 

Electromagnetic  motors  are  operated 
either  from  continuous  or  alternating-cur- 
rent circuits.  A  motor,  however,  which 
is  arranged  to  work  on  a  continuous-cur- 
rent circuit  is  not  usually  capable  of  oper- 
ating on  an  alternating-current  circuit,  and 
vice  versa.  Moreover,  in  either  case  a 
motor  will  only  operate  effectively  at  the 
E»  M.  F.  for  which  it  is  designed. 

A  particular  form  of  small  motor,  oper- 
ated from  110  volt  continuous-current  pres- 
sure, is  represented  in  Fig.  109.  31,  My  are 
the  field  magnets,  A,  the  armature,  CO, 
the  commutator,  B,  one  of  the  brushes  rest- 
ing upon  the  commutator,  and  P,  the 
pulley. 

Another  form  of  small  motor  intended  to 
be  driven  by  a  primary  or  storage  battery 
is  shown  in  Fisr.  110.     A  form   of  small 


ELECTRO-THERAPEUTICS. 


305 


Fig.  109.— Electromagnetic  Motor. 


306  -ELECTRICITY   IN 

alternator  designed  for  electro-therapeutic 
purposes  is  represented  in  Fig.  111.  The 
coils  O,  C,  C,  on  the  field  frame,  are  wound 


Fig.  110. — Electromagnetic  Motor. 

with  two  circuits  ;  a  coarse  wire  circuit 
excited  by  a  continuous  current  from  a 
pair  of  binding  posts  P,  and  a  fine  wire  cir- 
cuit connected  with  the  binding  posts  S. 
The  armature  A  A,  which  is  rotated  by  a 
small  pulley  at  one  end  of  the  shaft,  is  con- 


ELECTRO-T] 


structed  of  sheets  of  \o%  iron,  and  carries 
teeth  Tj  T,  in  such  a  mSijn^  ^alternately 
to  open  and  close  the  inM^i^t^rw?     7  >ifn 


Fig.  111.— Electro-Therapeutic  Alternator. 


the  coils  C,  C,  C.  There  are  12  poles  in 
the  field  frame,  so  that  each  revolution  of 
the  armature  produces  12  complete  periods, 
or  24  alternations.     As  soon  as  the  teeth 


308  ELECTRICITY   IN 

bridge  across  adjacent  poles,  magnetic  flux* 
is  poured  through  the  secondary  circuits,  or 
fine  wire  circuits,  inducing  in  them  an  E. 
M.  F.  in  one  direction,  and  as  soon  as  the 
teeth  pass  beyond  this  position,  the  mag- 
netic circuits  are  opened,  and  the  secondary 
coils  are  emptied  of  flux,  thus  inducing  an 
oppositely  directed  E.  M.  F.  The  advan- 
tage of  such  an  alternator  is  that  it  fur- 
nishes  alternating  E.  M.  Fs.  of  approxi- 
mately sinusoidal  type,  and  at  a  frequency 
which,  within  certain  limits,  is  under  con- 
trol. At  the  high  speed  of  4,800  revolu- 
tions per  minute,  or  80  revolutions  per 
second,  the  frequency  of  alternation  will 
be  80X12=960  complete  cycles,  or  1,920 
alternations  per  second;  while  at  lower 
speeds  the  frequency  mil  be  correspond- 
ingly reduced.  The  E.  M.  F.  obtainable 
from  such  a  machine  is  about  50  volts, 
The  E.   M.   F.   at  terminals  is,   however. 


ELECTRO-THERAPEUTICS.  309 

considerably  less  than  this  when  ordinary 
loads  are  applied. 

Alternating  currents  are  frequently  sup- 
plied from  electric  lighting  stations  to  con- 
sumption circuits  and  buildings,  at  a  com- 
paratively high  pressure,  1,000  or  2,000 
volts  effective  being  the  pressure  com- 
monly employed.  As  this  is  a  danger- 
ously high  pressure  to  handle,  it  is  never 
permitted  to  enter  a  house,  the  pressure 
being  reduced  at  some  point  outside  the 
house,  by  an  apparatus  called  a  step-dotvn 
transformer,  which  is  a  form  of  induction 
coil  in  which  the  primary  wire  contains  a 
greater  number  of  turns  than  the  second-, 
ary.  Alternating  currents  generated  by 
large  alternators,  placed  in  the  central  sta- 
tion, are  sent  through  the  primary  coils  of 
the  transformer,  usually  by  overhead  wires. 
The   secondary   coils   of  the  transformers 


310  ELECTRICITY   IN 

generate  a  pressure  of  50,  100  or  200  volts, 
according  to  circumstances,  and  wires  from 


Fig.  112.— Alternating  Current  Transformer. 

the  secondary  coil  enter  the  building 
to  be  supplied.  A  form  of  transformer 
is   shown   in   Fig.    112,  P,  P,  being   the 


ELECTRO-THERAPEUTICS.  311 

primary,  and  S,  S,  the  secondary  wires. 
The  ratio  of  the  secondary  to  the  primary 
pressure  is  called  the  ratio  of  transforma- 
tion. Thus,  if  the  primary  pressure  be- 
tween the  wires  P,  P,  be  1,000  volts  effec- 
tive, and  the  secondary  pressure  between 
the  wires  S,  Sf  50  volts  effective,  the  ratio 
of  transformation  is  1 :  20,  and  this  will  be 
approximately  the  ratio  of  the  number  of 
turns  in  the  secondary  coil  to  the  number 
of  turns  in  the  primary  coil  of  the  trans- 
former. The  frequency  of  alternation 
employed  in  alternating-current  electric 
lighting  is  not  higher  than  140~,  or  280 
alternations  per  second,  and  usually  varies 
between  this  and  125~  or  250  cycles.  In 
some  cases,  however,  the  frequency  may 
be  60 ~  and  even  as  low  25 ~  per  second. 

A  particular  form  of   transformer,    de- 
signed   for  supplying  alternating   electric 


312 


ELECTRICITY   IN 


currents  at  the  pressure  required  to 
operate  a  cautery  knife,  is  shown  in  Fig. 
113.     PP,  is  the  primary  coil  wound  upon 


Fig.    113. — Alternating-Current    Transformer  for 
Cautery. 


an  iron  core,  consisting  of  a  bundle  of 
straight  iron  wires  and  resembling,  there- 
fore, in  general  form,  the  primary  coil  of 
a  medical  induction  coil.  The  secondary 
coil  S,  consists  of  a  short  coil  of  thick  wire, 
which,    having  a   low   resistance,   enables 


ELECTRO-THERAPEUTICS.  313 

comparatively  powerful  currents  of  say  5 
to  30  amperes  to  be  produced  in  the 
secondary  circuit.  The  apparatus  is, 
therefore,  a  step-down  transformer.  The 
primary  coil  is  wound  for  an  effective 
alternating  pressure  of  50  or  100  volts, 
according  to  the  pressure  employed  in  the 
lighting  circuits  of  the  building.  In  order 
to  regulate  the  E.  M.  F.  and  current  in  the 
secondary  circuit,  the  secondary  coil  S,  is 
moved  from  or  towards  the  centre  of  the 
primary  coil  P,  by  the  screw  S,  after  the 
manner  of  the  Dubois-Rayinond  type  of 
adjustment  in  the  medical  induction  coil. 
The  contact  C\  is  so  arranged  that  by  the 
closing  of  the  box,  the  primary  circuit  is 
opened. 

In  some  cases,  where  continuous  cur- 
rents are  supplied  to  a  building  for  light- 
ing purposes,  at   110  volts  pressure,  it  is 


314 


ELECTRICITY   IN 


possible  to  dispense  entirely  with  the  use 
of  batteries  for  the  operation  of  a  medical 
i  lid  notion    coil,    or  for   the  production  of 


Fig.    114. — Adapter  for    Continuous  Current 
Circuits. 


feeble  continuous  currents  in  electro- thera- 
peutic work.  An  apparatus  for  this  pur- 
pose called  an  adapter  is  shown  in  Fig. 
114.     It  consists  essentially  of  a  rheostat, 


ELECTRO-THERAPEUTICS.  315 

placed  in  the  circuit  of  the  electric  light- 
ing; mains,  in  such  a  manner  as  to  reduce 
the  current  required  to  the  right  strength 
without  danger.  A  long  cylinder  RR  SSS, 
of  hard  rubber,  contains  at  the  end  RR,  a 
number  of  resistances,  which  are  connected 
with  brass  contact  strips  above.  The 
sliding  contact  C,  makes  connection  with 
one  of  these  brass  strips,  so  as  to  include 
any  desired  number  of  resistances  in  the 
circuit.  At  the  end  of  these  resistances, 
and  connected  with  thein,  is  a  long  spiral 
of  fine  German-silver  wire,  wound  in  a  fine 
groove  on  the  cylinder,  SS,  so  that  the 
contact  (7,  in  sliding  over  the  cylinder  to 
the  right,  makes  contact  in  succession  with 
each  turn  of  German-silver  wire,  thus  cut- 
ting out  the  resistance  very  gradually  over 
this  portion  of  the  circuit.  M,  is  a  milliam- 
metre,  and  I,  an  induction  coil,  whose  pri- 
mary circuit  is  operated  by  a  current  from 


316 


ELECTRICITY   IN 


the  electric    lighting    mains  through   the 
lamps  Z,  Z,  Z. 


The   connections    of    the    adapter    are 
shown  in  Fig.  115.     It  will  be  seen  that 


Fig.  115. — Connections  of  Adapter. 

the  current  through  the  mains  AB1  passes 
through  the  lamps  Zi,  through  the  circuit 
of  the  patient  at  P,  and  through  the  ad- 
justable resistance  and  milliammetre.  By 
connecting  the  middle  lamp  Z,  in  the  cir- 


ELECTRO-THERAPEUTICS.  317 

cuit,  the  pressure  connected  with  the 
patient  can  be  considerably  reduced.  It 
will  be  observed,  that  in  no  case  can  the 
circuit  of  the  patient  be  connected  to  the 
mains  without  the  interposition  of  two 
110-volt  electric  lamps.  The  primary  of 
the  induction  coil  can  be  thrown  into  cir- 
cuit by  the  use  of  the  switch  S. 

The  fact  that  an  incandescent  electric 
lamp  can  be  entirely  enclosed  in  a  non- 
conducting air-tight  glass  chamber,  renders 
it  suitable  for  introduction  into  the  cavi- 
ties of  the  body.  Two  miniature  incan- 
descent lamps,  suitable  for  such  explora- 
tory purposes,-- are  shown  in  Fig.  116. 
These  give  about  half  a  candle,  and  are 
operated  at  pressures  of  between  2  and 
4  volts,  with  a  current  strength  of  from  1 
to  1  1/2  amperes.  Care  must  be  taken  in 
the  operation  of  such  lamps,  that  the  pres- 


318  ELECTRICITY  IN 

sure  shall  not  exceed  that  for  which  they 
are  designed,  as  otherwise  the  lamps  will 
be  destroyed.  They  can  be  supplied  with 
either  an  alternating  or  a  continuous  cur- 
rent, but  are  usually  operated  by  a 
battery.      Since  a  1/2   candle-power  lamp 


i 


Fig.  116.— Incandescent  Electric  Lamps  for 
Exploration. 

requires  an  activity  of  about  3  1/2  watts, 
or  at  the  rate  of  7  watts  per  candle, 
while  a  lamp  of  8  candle-power  requires  to 
be  supplied  with  about  30  watts,  or  at  the 
rate  of  about  3  1/2  watts  per  candle,  it  is 
evident,  that  when  the  lamp  has  consider- 
able illuminating  power,  the  heat  it  liber- 


*  PRCFCEJY  (/ 

[cs.  319       v'f 


ELECTRO-THER\^EUTICS 

ates  may  be  inconvenientlj^gd^f,.,  .Conse- 
quently, when  the  candle-power  o^iamps^ 
for  exploratory  purposes  exceeds  a  certain 
amount,  it  is  customary  to  enclose  the 
globe  in  a  second  glass  chamber,  through 
which  water  is  circulated,  so  as  to  carry 
off  the  surplus  heat. 

The  heating  power  of  the  electric  cur- 
rent is  often  applied  in  surgery  for 
cauterizing  purposes.  Electric  cautery 
knives  consist  essentially  of  suitably 
shaped  platinum  wires,  heated  by  the  elec- 
tric current.  Fig.  117,  shows  several  forms 
of  such  cautery  knives.  The  amount  of 
activity  required  to  render  the  cautery 
knives  white  hot,  depends  upon  the  surface 
of  hot  platinum  which  they  expose  to  the 
air.  A  broad,  flat  knife  requires  more 
activity  than  narrow  blades.  Either  alter- 
nating or  continuous  currents  are  suitable 


320 


ELECTRICITY   IN 


for  cautery  knives.  Either  primary  or 
secondary  cells  are  frequently  employed 
for  this  purpose.     For  the  broadest  knife 


Fig.  117.— Electric  Cautery  Knives. 


in  the  figure,  25  or  even  30  amperes,  at 
a  pressure  of  approximately  one  volt,  may 
be  required,  representing  an  activity  in  the 
knife  of  from  25  to  30  watts.  In  the 
platinum  snare  cautery,  a  growth  or  part  is 


ELECTRO-THERAPEUTICS.  321 

removed  by  causing  a  length  of  wire  to  en- 
circle the  part  and  then  drawing  the  loop 
tight,  so  that  the  glowing  wire  is  pulled 
through  the  part  to  be  removed.  Here, 
owing  to  the  length  of  wire  which  has  to 
be  heated,  though  the  total  activity  may 
be  comparatively  small,  yet  the  E.  M.  F. 
necessary  to  send  the  required  current 
through  the  length  of  platinum  wire  may 
be  considerably  greater  than  that  for  a 
cautery  knife. 

We  have  seen,  that  in  accordance  with 
Ohm's  law,  the  current  strength  in  any 
circuit  may  be  altered,  either  by  varying 
the  E.  M.  F.,  or  by  varying  the  resistance. 
Both  of  these  methods  are  employed 
in  electro-therapeutics.  Instruments  for 
varying  the  resistance  in  a  circuit  are 
called  rheostats.  They  consist  essentially 
of  resisting  paths  whose  length  or  area  of 


322  ELECTRICITY  IN 

cross-section  may  be  adjusted,  or  varied  at 
will.  In  most  forms  of  rheostat,  it  is  the 
length  of  resisting  path  and  not  the  area  of 
cross-section,  which  is  varied. 

The  form  given  to  the  resisting  paths 
depends  upon  the  strength  of  the  current 
which  has  to  be  regulated.  Currents  for 
cauterizing,  which  may  be  as  high  as  20  or 
25  amperes,  require  comparatively  coarse 
wire  coils ;  for,  each  ohm  through  which  a 
current  of  25  amperes  passes,  liberates  heat 
at  the  rate  of  625  watts,  or  nearly  one 
H.  P.,  and,  consequent!}7,  if  this  one  ohm 
consisted  of  fine  wire  of  comparatively 
short  length  and,  therefore,  possessing  a 
very  limited  radiating  surface,  the  wire, 
being  unable  to  dissipate  this  heat,  would 
acquire  a  temperature,  probably,  sufficient 
to  melt  it.  The  comparatively  feeble  cur- 
rents    generally      employed     in     electro- 


ELECTRO-THERAPEUTICS.  323 

therapeutics  do  not  require  an  extensive 
radiating  surface,  and  the  rheostats  through 
which  they  pass  may  be  composed  of  fine 
wire  or  of  water  or  of  carbon. 


Fig.  118. — Carbon  Rheostat. 

One  of  the  simplest  forms  of  rheostat  for 
very  feeble  currents  is  shown  in  Fig.  118. 
Here  the  resisting  column  consists  of  a  thin 
layer  of  graphite  obtained  by  rubbing  a  soft 
graphite  pencil  in  a  circular  path  around 
the   rim    of  a   slate  slab,    CCC.     By   this 


324  ELECTRICITY   IN 

means  a  layer  of  fairly  high  resisting  car- 
bon is  obtained,  and  the  length  of  this 
path  in  a  circuit,  determines  the  amount 
of  resistance  included.  This  length  is  ad- 
justed by  altering  the  position  of  the  brush 
B,  attached  to  the  milled-headed  screw  M. 
It  becomes  necessary  in  practice,  to  occa- 
sionally renew  the  carbon  layer.  Its  resist- 
ance can  be  varied  by  rubbing  more  or 
less  graphite  over  the  surface. 

Another  form  of  carbon  rheostat  is 
shown  in  Fig.  119.  Here  the  resisting 
path  is  composed  of  pulverized  carbona- 
ceous material  pressed  into  a  groove  in 
an  insulating  plate.  A  number  of  brass 
studs,  CCO,  pass  through  the  surface  of 
the  insulating  plate  and  make  contact  with 
the  carbon  column  in  the  groove  beneath. 
The  length  of  the  carbon  column  inserted 
between  the  terminals,  T,  T,  can  be  varied 


ELECTRO-THERAPEUTICS. 


325 


by  turning  the  handle  H,  so  as  to  make 
contact  with  the  brass  studs  at  different 
portions  of  the  circumference. 


Fig.  119.— Carbon  Rheostat. 


Fig.  120,  shows  another- form  of  carbon 
rheostat  depending  upon  a  somewhat  dif- 
ferent principle.  Here  powdered  carbon 
is  placed  in  a  chamber  provided  with  elas- 
tic sides  CO.  The  resistance  between  the 
top  and  bottom   surfaces  of  this  mass  of 


ELECTRICITY   IN 


carbon  depends  upon  the  pressure  which 
is  brought  to  bear  upon  the  layer.  When 
the  pressure  is  very  light  the  carbon  par- 


Fig.  120. — Carbon  Pressure  Rheostat. 


tides  do  not  make  good  electric  contact 
with  each  other  and  interpose  a  compara- 
tively great  resistance  to  the  passage  of 
the  current  from   one  to  another.     When, 


ELECTRO-THERAPEUTICS. 


327 


however,  the  pressure  is  considerable,  the 
particles  are  brought  into  more  intimate 
electric  contact  and  the  resistance  of  the 


Fig.  121. — Water  Rheostat. 


mass  is  thereby  greatly  reduced.  The 
pressure  in  this  instrument  is  varied  by 
turning  the  milled-headed  screw  M. 

Fig*.     121   represents    a    form  of  water 


328  ELECTRICITY. 

rheostat.  Here  the  column  of  resisting 
material  is  composed  of  water,  Avhich,  as 
we  have  seen,  possesses  a  high  resistivity. 
The  binding  posts  bby  constituting  the 
terminals  of  the  instrument,  are  connected 
each  to  a  triangular  mass  of  carbon,  CO, 
armed  at  its  extremity  with  the  small 
sponge  S.  In  order  to  vary  the  resist- 
ance, the  milled  head,  M,  is  turned,  which 
by  means  of  a  worm  gear  rotates  the  car- 
bon plates  so  as  to  move  them  into  or  out 
of  the  liquid,  and  thus  vary  both  the 
length  and  cross-section  of  the  liquid 
column   between   them. 


CHAPTER  XII. 

HIGH    FREQUENCY    DISCHARGES. 

All  the  electric  sources  we  have  de- 
scribed produce  E.  M.  Fs.,  and  all 
E.  M.  Fs.,  when  permitted  to  do  so,  pro- 
duce electric  discharges  or  currents. 
The  type  and  magnitude  of  E.  M.  F. 
determine  the  type  and  magnitude  of  the 
electric  current.  It  is,  therefore,  to  be 
remembered  that,  however  different  may 
be  the  appearance  of  the  machine  which 
produces  an  electric  discharge,  or  however 
different  may  be  the  appearance  of  the 
discharge  itself,  the  difference  electrically 
is  simply  one  of  frequency,  magnitude  and 
wave  type  of  E.  M.  F. 


330  ELECTRICITY   IN 

A  high  E.  M.  R,  no  matter  how  pro- 
duced, may  discharge  in  three  ways. 

(1)  Convectively. 

(2)  Conductively. 

(3)  Disruptively. 

Either  of  the  two  last  mentioned 
methods  may  be  oscillatory  or  non-oscilla- 
tory. 

A  convective  discharge  is  the  discharge 
which  occurs  in  the  neighborhood  of 
points  connected  with  a  source  of  high 
electric  pressure.  A  pressure  of  20,000 
volts,  or  upwards,  will  produce  convective 
effects.  Such  a  pressure  is  furnished  by 
an  electrostatic,  or  influence  machine,  so 
that  if  an  upright  metallic  rod  S,  furnished 
with  a  sharp  pivot  point,  be  attached,  as 
shown  in  Fig.  122,  to  the  prime  conductor 
of  a  machine,  a  wheel,  formed  of  a  number 


ELECTRO-THERAPEUTICS. 


331 


of  radially  pointed  spokes,  supported  on 
the  pivot,  will  be  set  into  rapid  rotation  by 
the  reaction  of  the  convective  discharge  of 
electrified  air  particles,  that  are  thrown  oif 


Fig.  122.- 


-Rotation  Produced  by  Convective 
Discharge. 


from  the  points.  This  motion  of  the  air 
produces  a  breeze,  called  a  static  or  electric 
breeze,  which  is  sometimes  employed  elec- 
tro-therapeutically. 


If  a  damp  cord  be  made  to  connect  the 
main  terminals  of  a  high-pressure  ma- 
chine, a  silent  or  conductive  discharge  .will 


332'  ELECTRICITY  Iff 

pass  through  it,  the  resistance  offered  by- 
such  a  cord  being  a  very  great  number  of 
ohms.  It  might  be  supposed,  that  if  a  me- 
tallic wire  were  employed  instead  of  a 
string,  that  the  discharge  would  pass  more 
readily  than  it  would  through  a  conduct- 
ing string;  but,  curiously  enough,  this  is 
not  the  case,  owing  to  the  fact  that  the  low 
resistance  of  the  wire  causes  an  enormous 
current  strength  to  tend  to  flow  through 
it  under  a  high  pressure  at  its  termi- 
nals. Under  the  influence  of  this  enor- 
mous, rush  of  current,  the  inductance  or 
self-induction  of  even  a  short  length  of 
straight  wire,  is  sufficient  to  produce  a 
C.  E.  M.  F.  so  great,  that  a  disruptive  dis- 
charge may  take  place  across  a  consider- 
able air-gap,  before  any  appreciable  quan- 
tity can  escape  through  the  wire. 

When  a   knuckle   of   the   hand   is   ap- 


ELECTRO-THERAPEUTICS.  333 

proached  to  the  rounded  prime  conductor 
of  a  high -pressure  machine,  a  disruptive 
discharge  or  spark  will  pass  through  the 
air-gap,  between  the  hand  and  the  con- 
ductor. This  appears  to  consist  of  a  single 
discharge,  but  generally  consists,  in  reality, 
of  a  number  of  separate  discharges  to-and- 
fro  between  the  machine  and  the  hand. 
In  other  words,  the  discharge  is  oscillatory, 
and  the  current  oscillating.  The  difference 
between  a  quiet  steady  discharge,  of  a 
given  quantity  of  electricity  at  high  pres- 
sure, as  compared  with  an  oscillatory  dis- 
charge of  the  same  quantity,  is  shown  in 
Fig.  123.  At  A,  a  steady  discharge,  com- 
mencing at  say  100,000  volts  pressure,  falls 
steadily  to  zero ;  that  at  B,  starting  at  an 
equal  voltage,  falls  more  rapidly  to  zero 
and  is  slightly  oscillatory ;  that  at  C, 
rapidly  changes  direction  and  becomes 
oscillatory.     The  current  strength  in  the 


334 


ELECTRICITY   IN 


circuit  has  the  same  graphic  type  in  each 
case.  The  frequency  of  oscillation  of  these 
discharges  is  often  exceedingly  high,  reach- 
ing sometimes  hundreds  of  millions  of 
cycles  per  second.     The  total  number  of 


a 
\             A 

id 

\            B 

Ve 

\      j       ° 

\  \  \   C\    n   p 

/ 

\  /  Wo  e 
1  1  \J  m 

\\    k 

i 

Fig.  123.— Oscillatory  Discharge. 


oscillations,  however,  in  any  discharge  is 
not  very  great,  usually  varying  from  2 
or  3  to  20  or  30,  according  to  the  condi- 
tions of  the  circuit.  The  entire  discharge, 
therefore,  is  usually  completed  in  a  small 
fraction  of  a  second. 


ELECTRO-THERAPEUTICS. 


335 


If  a  steel  spring,  such  as  is  represented 
in  Fig.  124,  be  clamped  at  its  upper  ex- 
tremity, while  its  lower  end  is  loaded  with 


Fig.  124.— Mechanical  Vibrator,  Side  and  End  View. 


a  weight  W,  and  also  carries  a  vane  V, 
movable  in  a  viscous  liquid,  then,  if  the 
spring  be  drawn  aside  from  its  position 
of  rest,  to  the  position  S'  W  V ,  and  then 
released,  it  will  after  a  number  of  vibra- 


336  ELECTRICITY   IN 

tions  or  oscillations  return  to  rest,  in  a 
manner  which  will  depend  upon  the  fric- 
tional  resistance  offered  by  the  liquid,  upon 
the  elasticity  of  the  spring,  and  upon  the 
weight  with  which  it  is  loaded. 

If  the  friction al  resistance  of  the  liquid 
is  very  great,  relatively  to  the  elasticity  of 
the  spring,  such,  for  example,  as  might  be 
offered  by  impulses  to  tbe  motion  of  a 
large  vane  in  molasses,  then  the  spring 
will  not  oscillate,  but  will  slowly  return 
towards  its  position  of  rest.  If,  on  the 
other  hand,  all  frictional  resistance  could 
be  withdrawn,  not  only  in  the  vessel  of 
liquid,  but  also  in  the  air  and  in  the  mo- 
lecular structure  of  the  spring,  then  the 
spring  would  perform  oscillations  which 
would  continue  for  ever,  as  there  would 
then  be  no  means  for  dissipating  the 
energy  of  the  vibrating  system.     In  a  con- 


ELECTRO-THERAPEUTICS.  337 

dition  intermediate  between  the  preced- 
ing, that  is  to  say,  when  the  frictional 
resistance  offered  to  the  motion  is  appre- 
ciable, but  not  excessive,  the  spring  will 
execute,  by  reason  of  its  elasticity,  a  cer- 
tain number  of  oscillations  of  successively 
diminishing  amplitude  before  it  comes  to 
rest. 

The  frequency  of  the  oscillations  ex- 
ecuted by  the  spring  depends  upon  its 
elasticity,  and  the  weight  it  carries.  The 
weaker  the  spring ;  i.  <?.,  the  less  its  elastic 
force,  the  slower  the  vibrations  ;  the  greater 
the  load,  the  slower  the  vibrations.  In 
order,  therefore,  to  produce  a  high  fre- 
quency, we  require  a  very  stiff  spring ; 
i.  e.y  a  short  thick  spring,  and  a  very  small 
weight.  On  the  contrary,  for  very  low 
frequency  vibrations,  we  require  a  long 
and  thin  or  weak  spring,  with   a   heavy 


338  ELECTRICITY   IN 

load.  Provided  the  frictional  resistance  of 
the  liquid  is  not  sufficiently  great  to  check 
all  vibration,  the  amount  of  friction  will 
have  only  a  very  small  influence  upon  the 
frequency,  and  will  affect  only  the  number 
of  oscillations  performed,  before  the  system 
comes  to  rest.  In  other  words,  if  the  spring 
oscillates,  the  friction  can  only  damp  the 
system,  but  if  the  friction  exceeds  a  cer- 
tain quantity,  depending  upon  the  size  of 
the  spring,  its  elasticity  and  load,  then 
oscillation  will  be  impossible. 

Any  electric  circuit,  in  which  a  discharge 
suddenly  takes  place,  obeys  laws  which 
are  precisely  parallel  to  those  we  have 
above  indicated  in  relation  to  the  disturbed 
spring.  The  frictional  resistance  of  the 
liquid  corresponds  to  the  resistance  of  the 
electric  circuit  in  ohms.  The  weakness  of 
the  spring  corresponds  to  the  electrostatic 


ELECTRO-THERAPEUTICS.  339 

capacity  of  the  circuit,  or  its  capability  of 
behaving  as  a  condenser,  and  the  load  or 
weight  added  to  the  spring,  corresponds  to 
the  inductance  of  the  circuit ;  so  that  instead 
of  mechanical  inertia,  in  the  electric  circuit 
we  meet  electromagnetic  inertia.  When, 
therefore,  a  discharge  takes  place  in  an 
electric  circuit,  this  discharge  will  be 
oscillatory  or  non-oscillatory,  according  to 
the  amount  of  resistance  in  the  circuit 
relative  to  its  capacity  and  inductance. 
The  greater  the  resistance,  the  less  the 
probability  of  obtaining  an  oscillatory  dis- 
charge, and  when  the  resistance  is  very 
high,  the  discharge  takes  place  slowly  and 
without  oscillation.  If,  however,  the  re- 
sistance is  sufficiently  small  to  permit  oscil- 
lations or  alternations  of  current  to  take 
place  in  the  circuit,  then  the  resistance  will 
have  very  little  effect  upon  the  frequency 
of   alternation,  but   will   affect    only   the 


340  ELECTRICITY   IN 

damping  out  of  the  vibrations.  The  less 
the  resistance,  the  more  slowly  the  oscilla- 
tions will  die  out,  and  the  greater  the 
number  that  will  be  performed  before 
extinction.  In  the  same  way,  a  circuit  of 
large  electrostatic  capacity  behaves  like  a 
weak  spring  of  great  length,  and  a  circuit 
of  small  electrostatic  capacity,  like  a  small 
or  short,  stiff  spring. 

In  order  to  produce  very  rapid  oscilla- 
tion or  alternations  in  the  discharge  of  a 
circuit,  it  is  necessary  to  have  a  small  con- 
denser, and  a  small  inductance  in  the  cir- 
cuit. On  the  other  hand,  a  large  con- 
denser, discharging  through  a  circuit  of 
many  loops  of  wire,  and  having,  therefore, 
a  large  inductance,  will  perform  slow  oscil- 
lations or  oscillations  of  low  frequency. 
Unfortunately,  however,  for  the  production 
of  very  high  frequency  oscillations,  a  very 


ELECTRO-THtfKAPEl 


^pWsa^ 


small    condenser    onJ   ^contains  '  a^  kstriaB. 


&, 


quantity  of  electricity  f^£^given  pressui 
applied,  and,  consequentIj7^e^a%plitude^ 
of  the  current  strength  in  the  oscillations 
t  is  feeble,  and  the  total  amount  of  energy 
comparatively  small.  On  the  other  hand, 
a  large  jar,  which  will  hold  a  large  quan- 
tity of  electricity,  and  give  rise  to  power- 
ful oscillations,  can  only  produce  compara- 
tively low  frequency  currents.  The  fre- 
quency of  alternating-current  discharges, 
when  of  an  oscillating  character,  and  from 
an  ordinary  Leyden  jar  of  pint  size,  is 
roughly  about  15,000,000  of  periods  per 
second,  when  only  a  short  length  of  wire 
is  employed  to  connect  the  external  and 
internal  coatings. 

In  Pig.  125,  a  condenser  or  Leyden  jar 
J  is  represented  as  being  about  to  dis- 
charge  through   an   air-gap  in  its  circuit 


342 


ELECTRICITY   IN 


Jabc.  The  discharge  will  be  oscillatory 
if  the  resistance  for  the  circuit  be  suffi- 
ciently small.  Similarly,  if  as  shown  at  B, 
the  condenser  Cy  be  charged  by  turning  the 
switch  S,  to  a,  the  charge  from  the  E.  M.  % 
F.   will    be   oscillatory    if    the    resistance 


«U 


Fig.  125. — Circuit  for  the  Production  of  Alter- 
nating or  Oscillatory  Currents. 


of  the  charging  circuit  be  sufficiently  low. 
The  condenser  may  receive  a  non-oscilla- 
tory charge  and  give  an  oscillatory  dis- 
charge, or  vice  versa,  by  properly  propor- 
tioning the  resistance  of  the  circuits. 

The  E.  M.  F.  of  the  discharge  will,  of 
course,  be  greater,  the  greater  the  distance 
through  which  the  spark  discharge  passes, 


ELECTRO-THREEAPUTICS.  343 

but  whatever  the  E.  M.  F.,  the  frequency 
of  oscillation  in  the  circuit  will  be  the 
same,  and  only  the  amplitude  of  the 
waves  will  be  affected.  It  will  be  evi- 
dent, that  if  a  very  low  E.  M.  F.  be  em- 
ployed, the  waves  will  be  very  feeble  and 
a  high  E.  M.  F.,  such  as  supplied  by  an 
influence  machine,  is  necessary  for  power 
ful  oscillations. 

The  frequency  of  oscillation  in  a  given 
circuit  is  not  readily  computed  with  any 
degree  of  accuracy,  when  the  circuit  is 
very  short,  owing  to  the  fact  that  even  a 
straight  wire  offers  an  amount  of  induc- 
tance appreciable  to  rapid  discharges,  and, 
although  the  inductance  of  a  coil  of  many 
turns  of  wire  can  be  measured  or  calcu- 
lated, that  of  a  short  length  of  bent  wire 
is  difficult  to  estimate.  The  frequency  of 
oscillation  of  spark   discharges   has   been 


344  ELECTRICITY    IN 

experimentally  observed,  in  a  number  of 
cases,  by  observing  or  photographing  the 
succession  of  sparks  in  a  discharge  with 
the  aid  of  a  rapidly  rotating  mirror. 

It  is  possible,  therefore,  to  render  the 
charge  or  discharge  of  a  circuit  oscillatory 
by  suitably  regulating  its  capacity,  induc- 
tance and  resistance.  Such  discharges  are 
frequently  used  in  electro- therapeutics 
under  the  title  of  static  induced  currents, 
an  unfortunately  misleading  term.  The 
usual  connections  employed  in  such  a  cir- 
cuit are  shown  in  Fig.  126,  where  A  and 
B,  the  main  terminals  of  an  influence 
machine,  are  maintained  at  a  high  pressure 
until  they  discharge  through  an  interven- 
ing air-gap.  Before  discharge  occurs,  the 
Leyden  jars  j,  j,  become  charged  by  this 
pressure,  and  the  discharge  of  the  jars 
occurs  as  an  impulsive  discharge,  that  is  to 


ELECTRO-THERAPEUTICS. 


345 


say,  an  oscillatory  discharge.  The  number 
of  oscillations  in  the  discharge  will  depend 
upon  the  resistance  in  the  circuit,  both  of 


Fig.  126. — Oscillatory  Current  Circuit  of  Influence 
Machine. 


the  spark  gap  G,  and  the  resistance  H,  of 
the  patient  between  A  and  B.  The  resis- 
tance of  the  spark  gap  is  not  definitely 
known,  but .  from  the  observed  dampen- 
ing effect  in  experimental  circuits,  its  resis- 


346  ELECTRICITY   IN 

tance  appears  to  be  only  a  few  oliins.  The 
frequency  of  oscillations  depends  upon  the 
capacity  of  the  jars  and  the  inductance  of 
their  circuit.  Under  ordinary  conditions, 
the  frequency  is  several  hundred  thousand 
periods  per  second. 

In  order  that  oscillations  shall  be  set  up 
in  the  circuit  CD,  it  is  not  essential  that 
two  Leyden  jars  should  be  used,  although 
their  presence  assists  in  maintaining  the 
insulation  of  the  prime  conductors  of  the 
machine.  The  spark  discharge,  which 
occurs  at  the  air-gap  G,  will  always  be 
oscillatory,  provided  that  sufficient  capac- 
ity exists  connected  with  the  machine, 
relative  to  the  resistance  and  inductance  of 
the  discharging  circuit.  Two  equal  Ley- 
den jars,  in  series,  are  shown  in  the  figure, 
connected  as  a  single  Leyden  jar,  and 
equivalent  to  half  the  capacity  of  either, 


ELECTRO-THERAPEUTICS.  347 

As  already  stated  on  page  81,  the  pass- 
age of  one  coulomb  of  electricity  through 
a  circuit  in  one  second  of  time  means  a 
rate  of  flow,  or  current  strength,  of  one 
ampere,  on  the  average,  during  that  time. 
If  this  coulomb  passed  through  the  circuit 
in  one  thousandth  of  a  second,  the  mean 
current  strength  would  be  1,000  amperes, 
and  if  in  the  millionth  of  a  second,  the 
mean  current  strength  would  be  1,000,000 
amperes.  For  this  reason,  although  the 
total  quantity  of  electricity  in  a  pair  of 
Leyden  jars,  such  as  represented  in  Fig. 
122,  even  when  charged  at  a  pressure  of 
thousands  of  volts,  is  very  small,  yet, 
owing  to  the  great  frequency,  or  rapidity 
with  which  this  charge  is  jmssed  through 
the  circuit,  the  current  strength  during 
that  time  may  be  very  considerable.  The 
patient  placed  in  the  circuit  between  A 
and  B,  may,  therefore,  be  traversed  by  an 


348  ELECTRICITY  IN 

alternating  current  of  much  greater 
strength  than  he  would  be  able  to  receive 
without  pain  under  ordinary  conditions. 
For  reasons  which  are  yet  not  thor- 
oughly understood,  but  which  are 
believed  to  be  physiological  rather  than 
physical,  as  soon  as  a  certain  frequency  of 
alternation,  in  an  alternating  current  is 
attained,  the  sensory  effect  of  the  current 
almost  disappears,  as  though  it  required  a 
certain  interval  of  time  to  elapse  between 
successive  alternating  currents  for  a  nerve 
under  the  influence  of  this  current  to  register 
sensory  effects  in  the  brain.  It  would  seem 
probable,  therefore,  when  such  discharges 
of  high  frequency  pass  through  the  body, 
that  owing  to  their  high  current  strength, 
as  well  as  to  their  high  frequency,  power- 
ful physiological  effects  may  be  produced. 

So  far  as  is  at   present  known,  no  cur- 


ELECTRO-THERAPEUTICS.  349 

rent  can  pass  through  such  a  mass  of 
materials  as  that  of  which  the  human 
body  is  composed,  without  effecting  electro- 
lytic decomposition  or,  in  other  words,  that 
the  only  medium  of  conduction  in  such 
a  mass  is  chemical  decomposition  or  elec- 
trolysis. According  to  this  view,  rapidly 
alternating  currents  produce  electrolytic 
effects,  not  only  in  the  medium  immediately 
surrounding  the  poles  or  electrodes,  but 
also  in  the  intrapolar  or  intervening  tract. 

It  has  been  suggested  that  alternating 
currents  of  such  high  frequency  would  be 
unable,  in  traversing  the  human  body,  to 
penetrate  more  than  a  very  moderate  dis- 
tance below  its  surface,  and  that,  therefore, 
only  superficial  portions  of  the  body  could 
be  directly  affected  by  the  discharge. 
Owing,  however,  to  the  feeble  electric 
conductivity  of  the  materials  in  the  body, 


350  ELECTRICITY   II* 

this  shin  effect,  or  tendency  of  the  current 
to  seek  the  outer  layers  to  the  exclusion  of 
the  inner  layers,  is  comparatively  small  at 
the  frequencies  which  can  be  practically 
produced,  for,  while  the  depth  to  which 
such  currents  would  penetrate  in  good  con- 
ductors, such  as  copper  wires,  is  very 
small,  yet  in  the  case  of  comparatively 
high  resisting  materials,  such  as  those  con- 
stituting the  human  body,  the  penetration 
would  probably  extend  practically  through 
the  entire  mass.  High  frequency  alternat- 
ing currents  are,  therefore,  powerful  but 
painless  currents,  and  are,  probably,  at- 
tended by  electrolytic  effects  in  the  entire 
mass,  although,  as  in  the  case  of  all  alter- 
nating currents,  little  if  any  accumulation 
of  electrolytic  materials  can  take  place. 

It   is   not   necessary  to   employ  an   in- 
fluence  machine   for  purposes   of   obtain- 


ELECTRO-THERAPEUTICS.  351 

ing  high-frequency  alternating  currents. 
Alternating-current  dynamos  ;  i.  e.,  alter- 
nators, can  be  employed  to  produce 
directly  frequencies  up  to  10,000  periods 
per  second,  although  such  machines  are 
expensive  and  troublesome  to  operate,  re- 
quiring high  speeds  and  special  construc- 
tion. Powerful  induction  coils,  charging 
condensers,  are  also  capable  of  producing 
high  pressures  through  the  discharging 
circuit,  in  the  same  manner  as  influence 
machines. 

The  general  method  employed  for  pro- 
ducing high-frequency  alternating  cur- 
rents, is  illustrated  in  Fig.  127.  Here  S  S, 
is  the  secondary  winding  of  a  powerful 
induction  coil,  provided  with  a  spark  gap 
at  G.  S8,  is  preferably  excited  by  a 
low-frequency  alternating  current,  passing 
through  its  primary  coil,  say  for  example. 


352 


ELECTRICITY   IN 


from  an  alternating  electric  lighting  cir- 
cuit. The  function  of  s  s'y  is  to  produce 
high  pressures  which  charge  the  condenser 


mwmmm 


^Qo 


6s 


6s 


Fig.  127.— Apparatus  for  High  Frequency  Alternat- 
ing Currents. 


O,  with  a  quantity  of  electricity  propor- 
tional to  this  pressure.  The  spark  gap  G, 
is  so  adjusted  that  the  pressure  is  able 
to  discharge  across  it  and  in  such  dis- 
charge   to    permit    the    condenser    C,   to 


ELECTRO-THERAPEUTICS.  353 

empty  its  charge  through  the  circuit 
OppGC,  with  oscillations  depending  for 
their  frequency  upon  the  capacity  of  C\ 
and  the  inductance  in  the  circuit  compris- 
ing the  coil  pp.  This  coil  pp,  consists  of 
comparatively  few  turns  of  well-insulated 
wire,  and  serves  as  the  primary  winding  of 
an  induction  coil,  whose  secondary  SSf  has 
also  comparatively  few  turns,  although 
more  than  the  primary  pp1  but  the  turns 
are  very  carefully  insulated  from  each 
other.  The  effect  of  passing  these  very 
rapidly  alternating  currents  through  the 
primary  pp,  is  to  set  up,  by  mutual  induc- 
tion, very  powerful  induced  E.  M.  Fs. 
in  SS,  of  the  same  frequency,  which 
E.  M  Fs.  may  be  utilized  directly.  These 
very  powerful  induced  E.  M.  Fs.  are 
capable  under  suitable  conditions,  of  giv- 
ing sparks  several  feet  in  length,  and  these 
discharges,   representing    the   surgings    of 


354 


ELECTRICITY   IN 


hundreds  of  thousands  of  volts,  are,  never- 
theless, almost  painless,  owing  to  their 
high    frequency.     One    of    the    methods 


(WMumm 


Fig.  128. 


-Apparatus  for  Creating  Rapidly  Oscil- 
lating Magnetic  Field. 


which  have  been  applied  electro-therapeu- 
ticallv,  in  connection  with  high-frequency 
currents,  is  represented  in  Fig.  128.  Here 
the  secondary  winding  SS,  of  a  poweiTul 
induction  coil,  charges  and  discharges  the 


ELECTRO-THERAPEUTICS.  355 

condenser  C,  through  a  large  solenoid  or 
open  coil  Fy  of  comparatively  few  turns, 
upon  a  vertical  cylindrical  frame  about 
six  feet  high.  The  patient  is  introduced 
into  this  frame.  His  body  acting  as  a 
secondary  circuit,  induced  alternating  cur- 
rents circulate  around  it  generally  parallel 
to  those  in  the  primary  coil. 

When  the  best  results  are  desired  from 
high  frequency  discharge,  it  is  essential 
that  the  knobs  between  which  the  spark 
discharges  take  place,  shall  be  brightly 
polished.  If  this  precaution  is  not  taken, 
the  discharges  across  the  air-gap  are  apt  to 
assume  a  convective  rather  than  a  disrup- 
tive character. 


CHAPTER  XIII. 

ELECTROLYSIS     AND    CATAPHORESIS. 

The  more  modern  theory  of  electrolysis 
regards  the  conduction  of  electric  currents 
through  all  substances  except  metals  as 
a  convective  action,  in  which  only  free 
atoms  or  radicals  can  take  part ;  that  is  to 
say,  a  molecule  of  any  substance  is  in- 
capable of  conducting  electricity,  except 
in  the  case  of  metals.  Where  molecules, 
however,  are  dissociated  into  their  atomic 
constituents ;  i.  e.,  into  free  atoms  or  radi- 
cals, these  constituents  are  capable  of  re- 
ceiving and  conveying  electric  charges, 
and  so  become  the  medium  of  transport  in 
an  electric  current.     As  a  consequence  of 

356 


jR^ffE^ics.  357 

this,    the   atoms,    after  u^ving    oeliv'ei'ed!  * 
their    electric   charges,   acwrtalate  at  the 
electrode  to  which  they  are  oSfcspflf^ojs 

A  molecule  invariably  consists  of  two  dis- 
tinct parts  called  ions,  or  radicals,  named  re- 
spectively the  electro-positive  and  the  electro- 
negative ion  or  radical.  When  electrolytic 
decomposition  of  the  molecule  occurs,  it  is 
the  electro-positive  radical  or  ion  which 
appears  at  the  negative  electrode,  called 
the  cathode,  or  the  upway,  and  the  electro- 
negative radical  or  ion  which  appears  at 
the  positive  electrode,  called  the  anode,  or 
the  downway.  When,  for  example,  a  cur- 
rent is  led  between  platinum  electrodes, 
through  hydrochloric  acid,  it  is  supposed 
that  there  exists  in  the  liquid  besides  the 
hydrochloric  acid  molecules  proper,  a  con- 
siderable number  of  atoms  or  radicals  of 
hydrogen,  and  of  chlorine,  in  an  uncom- 


358  ELECTRICITY   IN 

bined  state,  resulting  from  the  decomposi- 
tion of  some  of  the  molecules.  When  an 
E.  M.  F.  is  connected  to  the  electrodes 
these  atoms  receive  electric  charges,  the 
hydrogen,  or  positively  charged  atoms, 
being  attracted  to  the  negative  electrode, 
and  the  chlorine,  or  negatively  charged 
atoms,  to  the  positive  electrode,  so  that  the 
passage  of  the  current  is  accompanied  by 
two  streams  of  ions  moving  in  opposite 
directions  through  the  liquid. 

When  a  quantity  of  electricity  passes 
through  a  liquid,  the  products  of  electro- 
lytic decomposition,  collected  at  the  elec- 
trodes, are  found  to  be  in  strict  proportion 
to  the  quantity  of  electricity  which  has 
passed.  Every  coulomb  of  electricity,  in 
its  passage  through  the  solution,  leaves  at 
the  electrodes  a  definite  number  of  ions 
differing  in  different  liquids.     Thus,  when 


ELECTRO-THERAPEUTICS.  359 

hydrogen  is  liberated,  this  quantity  is 
0.01038  railligranime-per-coulomb ;  so 
that  one  ampere;  i.  e.,  one  coulomb-per- 
second,  passing  through  a  solution  and 
liberating  hydrogen,  will  liberate  0.01038 
milligramme-per-second. 

A  certain  critical  value  of  the  E.  M.  F. 
is  required  between  the  electrodes,  in  a 
liquid,  before  electrolysis  can  take  place. 
In  other  words,  a  liquid  offers  a  certain 
C.  E.  M.  F.  having  a  definite  minimum 
value,  and  this  C.  E.  M.  F.  must  be  over- 
come, in  addition  to  the  C.  E.  M.  F.  due  to 
ohmic  resistance,  which  the  liquid  possesses 
by  virtue  of  its  resistivity  and  geometrical 
proportions,  before  the  current  will  pass 
through  the  liquid. 

When  liquids  that  are  capable  of  mix- 
ing, are   placed   in  a   vessel,  in   compart- 


360  ELECTRICITY   IN 

ments  separated  from  each  other  by 
porous  partitions,  an  unequal  mixing  of 
the  two  takes  place  through  the  pores  of 
the  partition  or  septum.  This  unequal 
mixing  through  the  pores  of  the  intercept- 
ing medium  is  called  osmose.  Under  its 
influence,  the  level  of  the  liquids  on  oppo- 
site sides  of  the  septum  will  be  changed. 
In  the  case,  for  example,  of  sugar  and  water, 
placed  on  one  side  of  the  septum,  formed  of 
say  a  piece  of  hog's  bladder,  and  pure  water 
placed  at  the  same  height  on  the  other  side, 
the  liquid  current  from  the  pure  water  is 
stronger  than  the  current  from  the  sugar 
and  water,  so  that  the  level  of  the  liquid 
rises  on  the  side  of  the  sugar  and  water. 
The  two  currents  are  sometimes  distin- 
guished as  follows  ;  viz.,  the  one  towards 
the  higher  level  is  called  the  endosmotic 
current,  and  the  one  toward  the  lower 
level,  the  exosmotic  current. 


ELECTRO-THERAPEUTICS.  361 

When  an  electric  current  is  sent 
through  a  porous  septum  separating  either 
two  different  liquids,  or  two  portions  of 
the  same  liquid,  some  of  the  liquid  is 
transported  bodily  through  the  septum, 
almost  always  in  the  direction  of  the  elec- 
tric current ;  that  is  to  say,  from  the  posi- 
tive pole  or  anode  toward  the  negative 
pole  or  cathode.  This  is  called  electric 
osmose  or  cataphoresis.  This  electric  os- 
mose takes  place  independently  of  ordinary 
osmose,  and  since  its  direction  varies  with 
the  current,  it  may  be  made  to  either  aid 
or  oppose  ordinary  osmose.  The  quantity 
of  liquid  transported  depends  both  on  the 
nature  of  the  liquid  and  on  the  nature  of 
the  porous  diaphragm,  but  in  every  case 
is  directly  proportional  to  the  quantity  of 
electricity  which  passes.  The  quantity 
transported  in  a  given  time  is,  therefore, 
proportional  to  the  current  strength.     The 


362  ELECTRICITY  IN 

thickness  and  area  of  the  porous  dia- 
phragm have  no  effect  upon  the  amount  of 
liquid  transported,  provided  the  current 
strength  is  constant,  but  it  is  evident,  that 
a  thick  septum  or  diaphragm  with  a  small 
active  surface,  will  add  a  greater  resist- 
ance to  the  circuit  than  a  thin  diaphragm, 
of  large  active  surface,  and,  consequently, 
will  tend  to  restrict  the  current  strength, 
and,  therefore,  the  amount  of  liquid  trans- 
ferred. With  any  given  porous  membrane 
the  rate  of  transfer,  or  the  quantity  of 
liquid  transferred  per-coulomb  of  electricity, 
is  directly  proportional  to  the  resistivity  of 
the  liquid  ;  the  higher  the  specific  resist- 
ance or  resistivity  of  the  liquid,  the  greater 
will  be  the  amount  of  liquid  transferred. 

For  the  above  reason,  a  very  dilute  so- 
lution of  a  salt  in  water  is  much  more 
rapidly  transferred  through  a  porous  dia- 


ELECTRO-THERAPEUTICS.  363 

phragm  by  electric  osmose  or  cataphoresis, 
than  a  dense  or  nearly  saturated  solution, 
since  the  dilute  solution  has  a  greater  re- 
sistivity, or  smaller  conducting  power,  but 
the  total  quantity  of  salt  transferred  in  a 
given  mass  of  dilute  solution  will  be  less 
than  that  transported  in  the  same  quantity 
of  concentrated  solution,  so  that  the 
advantage  of  employing  a  dilute  and 
rapidly  transported  solution  frequently  dis- 
appears. 

Since  the  human  skin,  from  a  physical 
point  of  view,  is  a  porous  diaphragm,  it  is 
possible  to  cause  almost  any  solution  to  be 
transferred  through  it  into  the  subjacent 
tissues,  by  placing  an  electrode  thoroughly 
moistened  with  the  solution  over  the  por- 
tion of  the  skin  selected,  and  connecting  it 
with  the  positive  terminal  of  the  source  of 
continuous  E.  M.  F.  employed,  while  the 


864  ELECTRICITY. 

negative  electrode  is  placed  in  contact  with 
some  other  portion  of  the  body.  Methods 
of  treatment  based  upon  this  action, 
whereby  drugs  or  medicaments  are  directly 
introducted  into  the  parts  to  be  acted  on, 
are  called  cataphoric  medication. 

The  combination  of  electrolysis  with 
cataphoric  medication  is  sometimes  called 
metallic  electrolysis.  Thus,  if  a  moistened 
copper  electrode  be  placed  over  a  surface 
of  skin,  or  mucous  membrane,  and  be  con- 
nected with  the  positive  pole  of  a  source  of 
continuous  E.  M.  R,  while  the  negative 
pole  is  placed  in  connection  with  some 
other  part  of  the  body,  a  salt  of  the  metal 
will  be  formed  at  the  surface  of  the  metal 
by  electrolysis,  and  this  salt,  entering  into 
solution  at  the  surface  of  the  skin,  will  be 
carried  through  the  skin  or  membrane  by 
cataphoric  action. 


CHAPTER  XIV. 

DANGERS   IN   THE   THERAPEUTIC   USE    OF 
ELECTRICITY. 

An  uninsulated  electric  conductor  carry- 
ing a  current,  though  held  in  the  hand,  is 
not  dangerous  from  the  passage  of  the  cur- 
rent, unless  the  wire  is  so  overheated  that 
the  wire  becomes  dangerously  hot.  If, 
however,  a  high  E.  M.  F.  be  connected 
with  the  wire,  then  holding  such  wire  in  the 
hand  may  become  dangerous,  if  a  circuit  be 
established  through  the  body  for  the  pas- 
sage of  an  electric  current  from  the  E.  M. 
F.  That  is  to  say,  the  body  may  receive 
a  dangerously  powerful  electric  current. 
A  man  standing  on  a  dry,  wooden  floor, 

365 


366  ELECTRICITY   IN 

may  safely  touch  a  wire  connected  with  a 
high  pressure.  For  example,  he  may  hold 
in  his  hand  a  bare  wire  leading  from  a 
dynamo  supplying  a  number  of  arc  lamps 
in  series,  and,  therefore,  having  a  differ- 
ence of  electric  potential,  relatively  to  the 
ground,  of  several  thousand  volts.  The 
man  will,  probably,  be  absolutely  uncon- 
scious of  any  effect  produced  by  the  current 
passing  through  the  wire.  But,  should 
the  man,  while  touching  this  wire,  come 
into  contact  with  some  other  electric  con- 
ductor, such,  for  example,  as  an  iron  beam 
connected  with  the  ground,  or  a  grounded 
wire,  he  may  receive  a  dangerously  power- 
ful, and  even  fatal,  electric  current  through 
his  body;  for,  if  a  ground  exists  any- 
where in  the  circuit  of  the  wire  he  holds, 
he  will  thus  permit  an  electric  circuit  to 
be  closed  through  his  body,  having  in  it  a 
considerable  E.  M.  F. 


ELECTRO-THERAPEUTICS.  367 

In  other  words,  merely  touching  at  one 
point  a  circuit  through  which  a  powerful 
current  is  passing,  is  not  sufficient  to  cause 
a  current  to  pass  through  the  body.  Not 
only  a  point  of  entrance,  but  also  a  point 
of  exit  and  a  complete  circuit  must  be 
provided  through  the  body,  before  a  cur- 
rent can  be  received.  It  is  for  this  reason, 
that  the  rule  is  frequently  adopted  in  elec- 
tric lighting  stations,  where  conductors 
carrying  high  pressure  currents  are  em- 
ployed, always  to  keep  one  hand  in  the 
pocket  when  touching  a  conductor.  By 
this  means,  if  the  floor  is  insulated,  it  will 
be  very  difficult  to  establish  a  circuit 
through  the  body. 

The  current  strength  which  will  be  re- 
ceived  by  the  body  under  any  given  condi- 
tions in  which  a  circuit  is  established  will, 
of  course,  depend  upon  the  E.  M.  F.  and 


368  ELECTRICITY   IN 

x>n  the  resistance  of  the  entire  circuit  in 
which  the  body  is  introduced,  according  to 
Ohm's  law.  The  resistance  of  the  human 
body  may  vary  enormously,  as  already 
pointed  out,  so  that  it  is  almost  impossible 
to  say  what  the  current  strength  will  be  in 
any  particular  case,  but,  generally  speak- 
ing, the  greater  the  surface  area  of  skin 
coming  into  contact  with  the  electrodes, 
and  the  moister  the  skin,  the  greater  will 
be  the  danger  of  receiving  a  fatal  shock 
from  a  powerful  E.  M.  F. 

Generally  speaking,  a  continuous  E.  M. 
F.  of  20  volts,  applied  anywhere  to  the 
human  body  through  the  unbroken  sur- 
face of  the  skin,  may  be  regarded  as  harm- 
less, since  the  current  strength  that  can  be 
made  to  pass  through  any  portion  of  the 
body  by  means  of  such  an  E.  M.  F.  is  very 
feeble.     Alternating  E.  M.  Fs.,  at  frequen- 


ELECTRO-THERAPEUTICS. 

cies  commercially  employed,  may  be  pain- 
ful under  certain  circumstances  at  pres- 
sures as  low  as  even  5  volts;  as,  for 
example,  when  the  hands  are  immersed  in 
a  jar  of  saline  solution,  and  these  jars  are 
connected  with  an  alternating  pressure  of 
5  volts  effective.  As  the  pressure  is  in- 
creased above  5  volts  of  alternating  E. 
M.  F.,  or  20  volts  of  continuous  E.  M.  F., 
the  physiological  effects  become  more 
painful,  and  the  continuance  of  such  a  cur- 
rent may  produce  serious  effects.  Fifty 
volts  of  alternating  E.  M.  F.  is  capable  of 
killing  a  dog,  in  two  or  three  seconds,  when 
suitably  applied  through  large  wet  elec- 
trodes, in  such  a  manner  as  to  meet  with 
a  comparatively  reduced  resistance  in  the 
body  of  the  animal. 

At  ordinary  commercial  frequencies,  it 
would     appear,    from     experiments    con- 


370  ELECTRICITY   IN 

ducted  upon  dogs,  horses  aud  cows,  that 
the  danger  of  a  given  alternating-current 
pressure  is  two  to  three  times  as  great  as 
that  of  the  same  amount  of  continuous-cur- 
rent pressure,  and,  moreover,  under  the 
action  of  a  powerful  alternating  current, 
the  animal  is  deprived  of  volitional  control 
of  its  muscles,  which,  are  thrown  into 
tetanic  rigidity,  a  much  greater  strength  of 
the.  continuous  current  being  necessary  to 
produce  a  similar  effect,  even  in  a  partial 
degree.  At  extremely  high  frequencies, 
however,  far  above  those  at  present  com- 
mercially employed,  we  have  seen  that  the 
physiological  effect  of  alternating  currents 
is  considerably  less  than  that  of  continu- 
ous currents  of  the  same  strength. 

Under  ordinary  circumstances,  a  man  re- 
ceives a  shock  from  a  wire  through  his 
hands  and  feet.     A  pressure  of  100  volts 


ELECTRO-T] 


Co 


e  than  appre- 


contmuous  is  not  rau 
ciable  when  the  hands  a 
same  may  be  said  of  50  volts  01  alterna- 
ting current.  A  pressure  of  500  volts  is 
capable  of  giving  a  very  severe  shock, 
especially  when  a  man  standing  on  the 
wet  ground,  touches  a  conductor  in  connec- 
tion with  a  trolley  wire,  at  about  500 
volts  pressure.  Rare  instances  are  said  to 
have  occurred  in  which  this  continuous 
current  pressure  has  been  fatal  to  man. 
Such  a  pressure  is  very  readily  capable  of 
killing  a  horse,  partly  owing  to  the  fact 
that  its  skin  is  almost  entirely  unpro- 
tected. It  would  also  appear  from  such' 
experimental  knowledge  as  we  possess  that 
animals  are  more  readily  killed  by  electric 
pressures  than  human  beings. 

The  current  strength  which  it  is  danger- 
ous to  employ  depends  both  upon  its  point 


c 


372  ELECTRICITY. 

of  application  and  upon  its  duration.  A 
current  of  250  milli amperes  is,  in  some  cases, 
harmless  when  conducted  through  por- 
tions of  the  human  body  for  a  short  inter- 
val of  time,  while,  in  other  cases,  this 
strength  of  current,  passed  through  vital 
organs,  might  produce  fatal  results.  In 
continuous-current  strength,  however,  any- 
excess  of  25  milliamperes  is  usually  at- 
tended with  pain  under  normal  conditions, 
and  is,  therefore,  regarded  as  a  strength  of 
current  that  should  only  be  administered 
with  due  precautions.  Even  this  strength 
of  current  through  delicate  organs,  such  as 
the  eye,  might  produce  serious  results. 


INDEX. 


Action  of  Electrified  Sphere,  Mechanical   Model 
of,  149,  150. 

Active  Conductor,  Magnetic  Flux  Paths  of,  192, 
193. 

■  Loop,  Influence  of,  on   Magnetic  Needle, 

193,  194. 

Activity,  Definition,  125. 

,  Electric,  Unit  of,  126. 

,  Mechanical,  Unit  of,  125,  126. 

Adapter  Connections  for  Continuous-Current  Cir- 
cuits, 316,  317. 

for  Continuous-Current  Circuits,  314,  315. 

Aero-Ferric  Magnetic  Circuit,  196. 

Alternating,  Current,  121. 

Current  Dynamo,  119. 

Current  Magneto-Electric  Generator,  246, 

247. 

373 


374  INDEX. 

Alternating-Current  Transformer,  309,  310. 

Current  Transformer  for  Cautery,  312,  313. 

E.  M.  F.,  113,  114. 

Alternation,  Definition  of,  117. 

Alternator,  Electro-Therapeutic,  307,  308. 

Alternators,  199,  299. 

Amalgam  for  Frictional  Electric  Machines,  141. 

Ammeter,  Definition  of,  90. 

Ampere,  81. 

,  Definition  of,  90. 

Ampere-Turn,  Definition  of,  207. 

Animal  Electricity,  Conclusions  in  Regard  to, 
9,  10. 

Anode,  357. 

Apparatus  for  High-Frequency  Alternating  Cur- 
rents, 352. 

Armature  of  Electromagnet,  204. 

B 

Battery,  Chloride  Storage,  57. 

of  Silver  Chloride  Cells,  39. 

,  Voltaic,  Definition  of,  50. 

,  Voltaic  Plunge,  52. 

Begohm,  Definition  of,  66. 
Bichromate  Voltaic  Cell,  41,  42. 
Bluestone  or  Gravity  Voltaic  Cell,  29,  30. 
Body,  Human,  Electric  Resistance  of,  76,  77,  78. 


INDEX.  375 

Body,  Human,  Electrolytic  Decomposition  Pro- 
duced in,  349. 

,  Human,   Heat  Produced  in  by  Different 

Current  Strengths,  135,  136,  137. 

Bonetti  Electrostatic  Machine,  180. 

Breeze,  Electric,  331. 

,  Static,  331. 

C 

C.  E.  M.  F.,  Produced  by  Chemical  Decom- 
position, 133. 

,  Produced  by  Magnetic  Activity,  133. 

,  Produced  by  Resistance  of  Circuit,  132. 

Calculation  of  Resistance,  69,  10. 

Calorie,  19. 

,  Lesser,  135. 

Calorimeter,  133,  134. 

Carbon  Pressure  Rheostat,  326. 

Rheostat,  323,  324. 

Cataphoresis,  361. 

and  Electrolysis,  356  to  364. 

Cataphoretic  Medication,  364. 

Cautery,  Alternating-Current  Transformer  for, 
312,  313. 

,  Electric,  Knives  for,  319,  320,  321. 

,  Platinum  Snare,  320,  321. 

Cell,  Charged,  54. 

,  Exhausted  or  Run  Down,  54. 


376  INDEX. 

Cell,  Primary,  Definition  of,  54. 

,  Secondary,  Definition  of,  54. 

,  Storage,  Chloride,  56,  57. 

,  Storage,  Definition  of,  54. 

,  Voltaic,  Bluestone  or  Gravity,  29,  30. 

,  Voltaic  Dry,  48,  49. 

,  Voltaic,  Exciting  Liquid  of,  30. 

,  Voltaic,  Leclanche,  27,  28. 

,  Voltaic,  Partz  Gravity,  46,  47. 

,  Voltaic,  Silver  Chloride  Form  of,  36, 37, 38. 


Cells,  Voltaic,  Double-Fluid,  30. 

,  Voltaic,  Single-Fluid,  30. 

— ' ,  Voltaic,  Various  Couplings  of,  88,  89. 

Charged  Cell,  54. 

Charging  Current,  55. 

Chemical  Decomposition  or  Electrolysis  Produced 

in  Human  Body,  349. 
Chloride  Storage  Battery,  57. 

Storage  Cell,  56,  57. 

Circuit,  Aero-Ferric,  196. 

,  Closed,  Definition  of,  34. 

,  Electric,  333. 

,  Electrostatic,  156. 

,  Ferric  Magnetic,  196. 

,  Magnetic,  191. 

,  Magnetic,    Character      and     Dimensions, 

Effect  of  Reluctance  of,  209,  210, 


INDEX.  377 

Circuit,  Magnetic  Methods  of  Varying  M.  M.  F. 

of,  212. 

,  Non-Ferric  Magnetic,  196. 

of  Alternating-Current  Transformer,  199. 

Classification  of  Electric  Sources,  26. 
Closed  Circuit,  Definition  of,  34. 
Coil,  Faradic,  248. 

,  Inducing,  233. 

,  Induction,  Simple  Form  of,  249. 

,  Primary,  234. 

,  Secondary,  234. 

Coils,  Faradic,  Adjustable  Vibrator  for,  274. 
Comb  of  Points  of  Frictional  Electric  Machine, 

141. 
Commutator,  Two-Part,  Diagram  of,  244,  245. 
Condenser,  Definition  of,  175,  176. 
Connections  for  Adapter,  316,  317. 

of  Medical  Induction  Coil,  290,  291. 

Contact  Theory,  Volta's,  6,  7. 
Continuous  Current,  121. 

Circuits,  Adapter  for,  314,  315. 

—  Dynamo,  107. 

Generators,  303. 

Continuous  E.  M.  F.,  107. 
Convective  Discharge,  330,  331,  332. 

Discharge,  Rotation  Produced  by,  321. 

Convention  as  to  Direction  of  Magnetic  Flux,  191. 


378  INDEX. 

Core  of  Medical  Induction  Coil,  267. 
Coulomb,  80. 

,  Micro,  147. 

per  second,  81. 

Counter  E.  M.  F.,  130. 
Couple,  Voltaic,  Definition  of,  30. 
Couplings,  Various,  of  Voltaic  Cells,  88,  89. 
Current,  Alternating,  121. 

,  Charging,  55. 

,  Continuous,  121. 

,  Direct,  122. 

,  Electric,  80  to  106. 

' — ,  Electric,  Definition  of,  80. 

,  Electrostatic,  156. 

,  Endosmotic,  360. 

,  Exosraotic,  360. 

,  Pulsating,  121. 

Strength,  Effective,  288. 

Strength  Employed  in  Electrocutions,  82. 

Currents,  Static-Induced,  344. 
Cycle,  Definition  of,  117. 


D 

D'Arsonval  Galvanometer,  105,  106. 
Dangers  in  the  Use  of  Electricity,  365  to  372. 


INDEX.  379 

Decomposition,  Chemical,  or  Electrolysis  Pro- 
duced in  Human  Body,  349. 

,  Electrolytic,  83,  34-9. 

Dielectric  Medium,  15V. 

Resistance,  167,  168. 

Direct  Current,  122. 

Direction  of  Induced  E.  M.  F.,  Rule  for,  225. 

Discharge,  Conductive,  331,  332. 

,  Convective,  330,  331. 

,  Convective,  Rotation  Produced  by,  321. 

,  Disruptive,  333. 

,  Impulsive,  344,  345. 

— ,  Oscillatory,  333. 

,  Silent,  331,  332. 

Discharges,  High -Frequency,  329  to  355. 

of  Medical  Induction  Coils,  Characteristics 

of,  298. 

Displacement,  Electric,  148.     » 

Lines,  157. 

Disruptive  Discharge,  333. 

Dissociation,  Molecular,  356. 

Dissymmetrical  Alternating  E.  M.  F.,  119. 

E.  M.  F.,  118. 

Double-Fluid  Voltaic  Cells,  30. 

Dr.  Ohm,  64. 

Dry  Voltaic  Cell,  48,  49. 

Voltaic  Cell,  E.  M.  F.  of,  49. 


380  INDEX. 

Dubois-Raymond  Type  of  Medical  Induction  Coil, 

261,  262. 
Dynamo,  Continuous-Current,  107. 
Dynamo-Electric  Generator,  299. 
Dynamos,  299. 

,  Alternating-Current,  119. 

and    Alternators,    Fundamental   Principle 

Involved  in   Production  of  E.  M.  F.  by, 

300,  301. 

,  Motors  and  Transformers,  299  to  328. 

,  Self -Exciting,  302. 

,  Separately-Excited,  302. 

E 

E.  M.  F.,  25. 

,  Alternating,  113,  114. 

and  not  Electricity  Produced  by  Electric 

Sources,  24',  25. 

,  Continuous,  107,  121. 

,  Continuous-Current  Dynamo,  110. 

,  Continuous,   Graphic   Representation    of, 

107. 

,  Counter,  130. 

,  Dissymmetrical,  118. 

,  Dissymmetrical  Alternating,  119. 

,  Effective,  288. 

,  Effective  Thermal,  288. 


INDEX.  381 

E.  M.  F.  in  Dynamos  and  Alternators,  Funda- 
mental Principle  Involved  in  Production 
of,  300,  301. 

,  Induction  of,  by  Magnetic  Flux,  221. 

,  Intermittent,  112. 

,  Methods  of  Discharge  of,  329,  330. 

,  Negative,  Graphic  Representation  of,  109. 

of   Continuous-Current   Dynamo,  Graphic 

Representation  of,  110. 

of  Dry  Cell,  49. 

of  Edison-Lalande  Cell,  44. 

of   Induction   Coil,   Methods   of  Varying 

Value  of,  250,  251. 

of  Partz  Gravity  Cell,  47. 

of  Self-induction,  Direction  of,  on  Break- 
ing Circuit,  229. 

of  Self-induction,  Direction  of,  on  Com- 
pleting Circuit,  229. 

of  Silver-Chloride  Cell,  38. 

of  Zinc-Carbon  Cell,  41. 

,  Positive,  Graphic  Representation  of,  107. 

Produced  by  Friction,  138,  139. 

Produced  by  Friction,  High  Value  of,  139, 

140. 

,  Pulsatory,  110. 

,  Sinusoidal,  121. 

,  Symmetrical,  118. 


382  INDEX. 

E.  M.  F.,  Symmetrical,  Wave  of,  119. 

,  Unit  of,  28. 

E.  M.  Fs.,  Franklinic,  144. 
Edison-Lalande  Cell,  E.  M.  F.  of,  44. 

Voltaic  Cell,  43,  44,  45. 

Effect,  Skin,  350. 

Effective  Current  Strength,  288. 

Thermal  E.  M.  F.,  288. 

Electric  Activity,  Source  of,  128,  12§. 

Breeze,  331. 

Calorimeter,  133,  134. 

Circuit,  333. 

Current,  80  to  106. 

Current,  Definition  of,  80. 

Displacement,  148. 

Osmose,  361. 

Resistance,  63  to  79. 

Resistance  of  Flesh,  75. 

Resistance  of  Human  Body,  76,  77,  78. 

Sources,  Classification  of,  26. 

Unit  of  Work,  125. 

Electricity    and    Magnetism,    Relation    Between, 

184,  185. 
and  Magnetism,  Transmission  of,  Through 

Vacua,  20,  21,  22. 

,  Animal,  Conclusions  in  Regard  to,  9,  10. 

,  Decomposition  by,  83. 


INDEX.  383 

Electricity,  Nature  of,  13,  14. 

,  Unit  of  Quantity  of,  80. 

Electrocutions,    Current   Strengths   Employed  in, 

82. 
Electrode,  Negative,  35 7. 

,  Positive,  357. 

Electrodes,  357. 

Electrolysis  and  Cataphoresis,  356  to  364. 

,  Definition  of,  83. 

,  Metallic,  364. 

Electrolyte,  Definition  of,  30. 
Electrolytic  Decomposition,  83,  349. 
Electromagnet,  200,  201. 

,  Aero-Ferric  Circuit  of,  200,  201. 

,  Horse-Shoe,  202,  203. 

,  Yoke  of,  204. 

Electromagnetic  Induction,  237,  238. 

Inrush,  338,  339. 

Motors,  304,  307. 

Electromotive  Force,  13  to  62. 

Force,  Abbreviation  of,  25. 

Force,  Nature  of,  24,  25. 

Force,  Varieties  of,  107  to  123. 

Electro-Negative  Ions,  357. 
Electropliorus,  Description  of,  166. 

,  Operation  of,  167  to  171. 

Electropoion  Fluid,  41. 


384  INDEX. 

Electro-Positive  Ions,  35 7. 

Electrostatic   Attraction    and  Repulsion,  General 

Laws  of,  164,  165,  166. 

Circuit,  156. 

Circuit,  Application  of  Ohm's  Law  to,  156. 

Circuits  of  Toepler-Holtz  Machine,  173. 

Current,  156. 

Flux,  148. 

Flux,  Line  or  Curves  of,  148. 

Flux  Paths,  Representation  of,  160. 

Induction,  144,  145,  146,  159. 

Law,  General,  of  Attraction  and  Repulsion, 

164,  165,  166. 

Resistance,  156. 

Electro-Therapeutic  Alternator,  307,  308. 
Electro-Therapeutics,  Galvani's  Contribution  to,  9. 
Elements,  Voltaic,  31. 

,  Voltaic,  Varieties  of,  33. 

Endosmotic  Current,  360. 
Ether,  Luminiferous,  19,  20. 

,  Transmission  of  Heat  by,  16,  17,  18. 

,  Universal,  14. 

Exciting  Liquid  of  Voltaic  Cell,  30. 

Exhausted  or  Run  Down  Cell,  54. 

Exosmotic  Current,  360. 

External  Damping  Tube  for  Induction  Coil,  281, 

282. 


INDEX.  385 

F 

Faradic  Coil,  248. 

Coil,  Adjustable  Vibrator  for,  274,  2*75. 

Ferric  Magnetic  Circuit,  196. 

Flesh,  Electric  Resistance  of,  75. 

Flow,  Electric,  Unit  of  Rate  of,  81. 

Fluid,  Electropoion,  41. 

Flux  Density,  213. 

,  Electrostatic,  148. 

,  Magnetic,  190. 

,  Magnetic,   Apparent   Failure  to  Produce 

Physiological  Effects  on  Human  Body, 
214,  to  218. 

,  Magnetic,  Induction  of  E.  M.  F.  by,  221 

to  247. 

,  Passage  of  through    Human   Body,   218, 

219,  220. 

Paths,  Effect  of  Shape  of  Body  on  Direc- 
tions of,  151  to  155. 

Paths,  Electrostatic  Representation  of,  160. 


-,  Remanent,  202. 
-,  Residual,  201. 


Foot-pound,  Definition  of,  124. 

per  second,  Definition  of,  125. 

Force,  Electromotive,  13  to  62. 

,  Electromotive,  Abbreviation  of,  25. 

,  Electromotive,  Nature  of,  24,  25. 


386  INDEX. 

Force,  Electromotive,  Varieties  of,  107  to  123. 

,  Magneto-motive,  206. 

,  Magneto-motive,  Unit  of,  207. 

Franklin,  144. 
Franklinic  E.  M.  Fs.,  144. 
Frequency,  Definition  of,  118. 

of  Oscillation,  337,  338. 

Friction,  Development  of  E.  M.  F.  by,  138,  139. 
Frictional  and  Influence  Machines,  138  to  184. 

Electric  Machine,  Comb  of  Points  of,  141. 

Electric  Machine,  Plate  Form  of,  141,  142. 

Electric  Machines,  140,  141. 

- Electric  Machines,  Amalgam  for,  141. 

Electric  Machines,  Rubber  of,  141. 

Frog,  Galvanoscopic,  2. 

G 

Galvani,  Discovery  of,  1  to  5. 
Galvanometer,  D'Arsonval,  105,  106. 

,  Mirror,  99,  100. 

,  Mirror,  Sensitive,  103,  104,  105. 

Galvanoscopic  Frog,  2. 

Gauss,  Definition  of,  213. 

Generator,  Alternating  Magneto-Electric,  246,  247. 

,  Dynamo-Electric,  299. 

,  Magneto-Electric,  239. 

Generators,  299. 


INDEX.  387 

Generators,  Continuous- Current,  303. 

Gilbert,  Definition  of,  207. 

Graphic  Representation  of  Continuous  E.  M.  F., 

107. 
Representation  of   Oscillatory    Discharge, 

334. 
Gravity  or  Bluestone  Voltaic  Cell,  29,  30. 
Grenet's  Voltaic  Cell,  41,  42. 
Grid  of  Storage  Cell,  56. 

H 

Heat,  Transmission  of,  by  Ether,  16,  17,  18. 

High-Frequency  Alternating-Currents,  Apparatus 
for,  352. 

Discharges,  329  to  355. 

Discharges,  Physiological  Effects  of,  347, 

348. 

Electric  Oscillations,  Conditions  Requisite 

for,  340,  341. 

Holtz  Influence  Machine,  Form  of,  179. 

Horse-Shoe  Electromagnet,  202,  203. 

Human  Body,  Electric  Resistance  of,  76,  77,  78. 

Body,  Electrolytic  Decomposition  Pro- 
duced in,  349. 

Body,    Heat    Produced    in,    by   Different 

Current  Strengths,   135,   136,   137. 

Body,  Passage  of  Flux  through,  218,  219, 

220. 


388  INDEX. 

I 

Impulsive  Discharge,  344,  345. 
Impurities,  Effect  of,  on  Resistivity,  71,  72. 
Incandescent  Lamps    for    Exploratory  Purposes, 

317. 
Induced  E.  M.  F.,  Direction  of,  225. 
Inducing  Coil,  233. 
Inductance,  332. 

of  Secondary  of  Induction  Coil,  264. 

Induction  Coil,  External  Damping  Tube  for,  281, 

282. 

— Coil,  Medical,  248  to  298. 

Coil,  Rapid  Interrupter  for,  278,  279. 

Coil,  Ribbon  Vibrator  for,  276,  277. 

Coil,  Simple  Form  of,  249. 

Coil,  Internal    Damping    Tube   For,  281, 

282. 
Coils,  Medical,  Relative   Effectiveness  of, 

285,  286. 

,  Electromagnetic,  237,  238. 

,  Electrostatic,  144,  145,  146,  159. 

,  Magneto-Electric,  238,  239. 

,  Mutual,  232  to  235. 

of  E.  M.  F.  by  Magnetic  Flux,  221  to  247. 

of  E.  M.  F.  by  Magnetic  Flux,  Varieties 

of,  221. 


ind: 


7* 

Induction  of  E.  M.  F.,  Me 

226,227,  228. 

Influence  Machine,  144 

Machine,  a  form  of  Electrophorus,  169, 170. 

Machine,    Oscillatory-Current    Circuit  of, 

345,  346. 

Inrush,  Electromagnetic,  338,  339. 

Insulators,  68. 

Intensity,  Magnetic,  Unit  of,  213. 

Interconnection  of  Primary  and  Secondary  Wind- 
ings of  Medical  Induction  Coil,  293,  294. 

Intermittent  E.  M.  F.,  112. 

Internal  Damping  Tube  for  Induction  Coil,  280. 

Ions  or  Radicals,  357. 

j 

Jar,  Leyden,  176,  177. 

Joint  Resistance,  73. 

Joule,  Definition  of,  125,  126. 

per  second,  Definition  of,  126. 

Julien  Storage  Battery,  59. 

K 

Kathode,  357. 

Knives  for  Electric  Cautery,  319,  320,  321. 


390  INDEX. 

L 

Lamps,  Incandescent,  for   Exploratory   Purposes, 

317. 
Law,  General,   of    Electrostatic    Attraction    and 

Repulsion,  164,  165. 

,  Ohm's,  84  to  90. 

Leclanche  Cell,  E.  M.  F.  of,  28. 

Voltaic  Cell,  27,  28. 

Lesser  Calorie,  135. 

Ley  den  Jar,  176,  177. 

Jar   Discharge,  Oscillatory  Character   of, 

341. 
Light,  Nature  of,  23,  24. 
,  Transmission  of,  by  Luminiferous  Ether. 

19,  20. 
Lines,  Displacement,  157. 

or  Curves  of  Electrostatic  Flux,  148. 

Luminiferous  Ether,  19,  20. 

M 

M.  M.  F.,  206. 

of  Circuit,  Methods  of  Varying  Value  of, 

212. 
Machine,  Frictional  Electric,  140,  141. 
Machines,  Frictional  and  Influence,  138,  183. 
Magnet,  North  Pole  of,  191. 
,  South  Pole  of,  192. 


INDEX.  391 

Magnetic  Circuit,  191,  192. 

Circuit,  Ohm's  Law  applied  to,  207. 

Circuit,  Varieties  of,  196. 

Field,  Rapidly  Oscillating,  Apparatus  for 

Producing,  354,  355. 

Flux,  190. 

Flux,     Apparent      Failure      to     Produce 

Physiological  Effects   on   Human  Body, 

214  to  218. 

Flux,  Convention  as  to  Direction  of,  191. 

Flux,  Induction    of    E.    M.  F.  by,  221  to 

247. 

—  Flux  Paths  of  Active  Conductor,  192,  193. 

Flux,  Unit  of,  211. 

Intensity,  Unit  of,  213. 

Needle,  Influence  of  Active  Loop  on,  193, 

194. 

Reluctance,  206. 

Resistance,  206. 

Magnetism,  184  to  220. 

and   Electricity,  Relation   Between,  184, 

185. 

and  Electricity,  Transmission  of,  through 

Vacua,  20,  21,  22. 

,  Definition  of,  184. 

Method  of  Producing,  188,  189. 

,  Permanent,  201. 


392  INDEX. 

Magnetism,  Residual,  202. 

Magneto-Electric  Generator,  239. 

Generator  changes  in  Magnetic  Circuit  of, 

241,  242. 

Induction,  238,  239. 

Magneto-Motive  Force,  206. 

Force,  Unit  of,  207. 

Mechanical  Analogue  of  Induction  of  E.  M.  F., 
226,  227,  228. 

Analogue  of  Relation  Between  Electricity 

and  Magnetism,  185  to  188. 

Model   of  Action   of   Electrified   Sphere, 

149,  150. 

Vibrator,  335,  336. 

Medical  Induction  Coil,  248,  298. 

Induction  Coil,  Characteristics  of  Dis- 
charge Produced  by,  298. 

Induction  Coil,  Connection  of  Vibrator  in, 

269  to  275. 

Induction  Coil,  Connections  of,  290,  291. 

Induction  Coil,  Core  of,  267. 

Induction  Coil,  Diagram  of  Primary  In- 
duced E.  M.  Fs.,  259. 

Induction  Coil  Discharges,  Characteristics 

of,  298. 

Induction    Coil,   Dubois-Raymond   Type, 

261,  262. 


INDEX.  393 

Medical  Induction  Coil,  Effect  of  Increasing  Fre- 
quency of  Vibration, 

Induction  Coil,  Interconnection  of  Primary 

and  Secondary  Windings  of,  293,  294. 

Induction     Coil,   Methods    of    Increasing 

Frequency  of  Flux  Oscillations  Pro- 
duced  by,    252. 

Induction     Coil,   Methods    of    Increasing 

Magnetic  Flux  of,  252. 

Induction  Coil,  Operation  of,  254,  258. 

Induction    Coil,  Primary  Connections  of, 

253,  254. 

—  Induction  Coils,  Relative  Effectiveness  of, 

285,  286. 

Medication,  Cataphoretic,  361. 

Medium,  Dielectric,  157. 

Megohm,  Definition  of,  66. 

Metallic  Electrolysis,  364. 

Milliameter,  Construction  of,  94  to  98. 

,  Definition  of,  90. 

,  Varieties  of,  91  to  94. 

Milliampere,  Definition  of,  82. 

Mirror,  Galvanometer,  99  to  102. 

,  Galvanometer,  Sensitive,  103,  104,  105. 

Molecular  Dissociation,  356. 

Motors,  Dynamos  and  Transformers,  299  to 
328. 


394  i:ntdex„ 

Motors,  Electromagnetic,  304  to  307. 
Mutual  Induction,  232  to  235. 

N 

Nature  of  Electricity,  13,  14. 

Negative  E.  M.  F.,  Graphic  Representation  of,  109. 

Electrode,  357. 

Plate  of  Voltaic  Cell,  31. 

Pole  of  Voltaic  Cell,  34. 

Non -Ferric  Magnetic  Circuit,  196. 
Non-Polarizable  Voltaic  Cells,  32. 
North  Pole  of  Magnet,  191. 

o 

i 
Oersted,  Definition  of,  211. 

Ohm,  Definition  of,  65,  66. 

Ohm,  Dr.,  64. 

Ohm's  Law,  84  to  90. 

Law,  Application  of  to  Electrostatic  Cir- 
cuit, 156. 

Law,  Application  of  to  Magnetic  Circuit, 

207. 

Oscillations,  Frequency  of,  337,  338. 

,    High-Frequency,      Electric     Conditions 

Requisite  for,  340,  341. 

Oscillatory  Character  of  Leyden  Jar  Discharge,  341. 


INDEX.  395 

Oscillatory  Current  Circuit  of  Influence  Machine, 
345,  346. 

Discharge,  333. 

Discharge,  Graphic  Representation  of,  334. 

Osmose,  360. 
,  Electric,  361. 


Pair,  Voltaic,  Definition  of,  30. 
Parallel-Connected  Resistances,  73. 
Partz  Gravity  Voltaic  Cell,  46,  47. 
Period,  Definition  of,  117. 
Permanent  Magnetism,  201. 

Physiological   Effects    of    High-Frequency    Dis- 
charges, 347,  348. 
Plate  Form  of  Frictional  Electric  Machine,  141. 
Platinum  Snare  Cautery,  320,  321. 
Plunge  Battery,  Voltaic,  52. 
Polarization  of  Voltaic  Cell,  32. 
Pole,  Negative,  of  Voltaic  Cell,  34. 

,  Positive,  of  Voltaic  Cell,  34. 

Portable  Silver-Chloride  Battery,  53. 

Positive  E.  M.  F.,  Graphic  Representation  of,  109. 

Electrode,  357. 

Plate  of  Voltaic  Cell,  31. 

Primary  Cell,  Definition  of,  54. 


396  INDEX. 

Pulsating  Current,  121. 
Pulsatory  E.  M.  F.,  110. 

R 

Radicals,  Electro-Negative,  357. 

,  Electro-Positive,  357. 

or  Ions,  357. 

Rapid  Interrupter  for  Induction  Coil,  278,  279. 
Rapidly   Oscillating   Magnetic   Field,   Apparatus 

for  Producing,  354,  355. 
Ratio  of  Transformation,  311. 
Reluctance,  Effect  of  Character  and  Dimensions  of 
•     Circuit  on,  209,  210. 

,  Magnetic,  206. 

of  Human  Body,  218,  219,  220. 

,  Unit  of,  211. 

Reluctivity,  208. 
Remanent  Flux,  202. 
Residual  Magnetism,  201,  202. 
Resistance,  Calculation  of,  69,  70. 

,  Dielectric,  167,  168. 

,  Electric,  63  to  79. 

,  Electric,  Definition  of,  63,  64. 

,  Electric,  of  Flesh,  75. 

,  Electric,  of  Human  Body,  76,  77,  78. 

,  Electrostatic,  156. 

,  Joint,  73. 


INDEX.  397 

Resistance,  Magnetic,  206. 

,  Specific,  67. 

,  Unit  of,  Electric,  64. 

Resistances,  Parallel-Connected,  73. 

,  Series-Connected,  72. 

Resistivities,  Effect  of  Temperature  on,  71. 

,  Table  of,  68. 

Resistivity,  Definition  of,  67. 

,  Effect  of  Impurity  on,  71,  72. 

of  Water,  71. 

Rheostat,  Carbon,  323,  324,  325. 

,  Carbon  Pressure,  326. 

,  Water,  327,  328. 

Rheostats,  321  to  328. 

Ribbon  Vibrator  for  Induction  Coil,  276,  277. 
Rubber  of  Frictional  Electric  Machines,  141. 
Rule  for  Direction  of  Induced  E.  M.  F.,  225. 

S 
Scale,  Mirror,  Galvanometer,  102,  103.' 
Secondary  Coil,  234. 

Induced    E.  M.  F.  of  Medical  Induction 

Coil  at  High  Frequency  under  Load,  273. 

of  Induction  Coil,  Inductance  of,  264. 

or  Storage  Cell,  Forms  of,  55  to  62. 

Self-exciting  Dynamos,  302. 
Self-induction,  222,  229,  230,  332. 


398  INDEX. 

Self-induction,  Counter  Electromotive   Force  of, 

229,  230. 
Sensitive  Mirror  Galvanometer,  102,  103,  104. 
Separately-Excited  Dynamos,  302. 
Series-Connected  Resistances,  72. 
Series  Connection  of  Voltaic  Cells,  50. 
Short  Circuit,  Definition  of,  87. 
Silent  Discharge,  331,  332. 
Silver-Chloride  Cell,  E.  M.  F.  of,  38. 

Cells,  Battery  of,  39. 

Portable  Battery,  53. 

■ Voltaic  Cell,  36,  37,  38. 

Single-Fluid  Voltaic  Cells,  30. 
Sinusoidal  E.  M.  F.,  121. 

Wave,  120,  121. 

Sources,  Electric,  Classification  of,  26. 

Skin  Effect,  350. 

Snare,  Platinum,  Cautery,  320,  321. 

South  Pole  of  Magnet,  191. 

Sparking  Distance  Through  Air-Gap,  140. 

Specific  Resistance,  67. 

Static  Breeze,  331. 

— Induced  Currents,  344. 

Step-Down  Transformer,  309. 
Storage  Cell,  Chloride.  56,  57. 

Cell,  Definition  of,  54. 

Cell,  Grid  of,  56. 


INDEX.  399 

Storage  Cell,  Julien,  59. 

— - —  or  Secondary  Cells,  Forms  of,  55  to  62. 

Symmetrical  E.  M.  F.,  118. 

Wave  of  E.  M.  F.,  119. 

T 

Table  of  Resistivities,  68. 

Temperature,  Effect  of  on  Resistivities,  71. 

Therapeutic  Uses  of  Electricity,  Dangers  in,  365 

to  372. 
Toepler-Holtz  Influence  Machine,  Construction  of, 

172. 

— Machine,  Operation  of,  173,  174,  175. 

Transformation,  Ratio  of,  311. 
Transformer,  Alternating-Current,  309,  310. 

,  Step-Down,  309. 

Transformers,  Motors  and  Dynamos,  299  to  328. 

Tregohm,  Definition  of,  66. 

Two-Part  Commutator,  Diagram  of,  244,  245. 

u 

Unit  of  E.  M.  F.,  28. 

of  Electric  Activity,  126. 

of  Electric  Resistance,  64. 

of  Electric  Work,  126. 

of  Magnetic  Flux,  211. 


400  INDEX. 

Unit  of  Magnetic  Intensity,  213. 

of  Magneto-Motive  Force,  207. 

of  Mechanical  Activity,  125. 

of  Quantity  of  Electricity,  80. 

of  Rate  of  Electric  Flow,  81. 

of  Reluctance,  211. 

of  Work,  124. 

Universal  Ether,  14. 

v 

Varieties  of  Electromotive  Force,  107  to  123. 

of  Magnetic  Circuit,  196. 

Vibrator,  Adjustable  for  Farad ic  Coil,  274. 

— ■ ,  Mechanical,  335,  336. 

Volt,  Definition  of,  28. 

Volta,  6. 

Volta's  Contact  Theory,  6,  7. 

Voltaic  Battery,  Definition  of,  50. 

Voltaic  Cell,  Bi-Chromate,  41,  42. 

Cell,  Edison-Lalande,  43,  44,  45. 

Cell,  Elements  of,  31. 

Cell,  Exciting  Liquid  of,  30. 

Cell,  Grenet,  41,  42. 

Cell,  Leclanche,  27,  28. 

Cell,  Negative  Plate  of,  31. 

Cell,  Partz  Gravity,  46,  47. 

Cell,  Polarization  of,  32. 

Cell,  Positive  Plate  of,  31. 


INDEX.  401 

Voltaic  Cell,  Silver  Chloride  Form  of,  36,  37,  38. 

Cells,  Connection  of,  in  Series,  50. 

Cells,  Double-Fluid,  30. 

Cells,  Non-Polarizable,  32. 

; —  Cells,  Single-Fluid,  30. 

Cells,  Zinc-Carbon,  40,  41. 

Couple,  Definition  of,  30. 

Dry  Cell,  48,  49. 

Elements,  31. 

Elements,  Varieties  of,  33. 

Pair,  Definition  of,  30. 

Volt-Coulomb,  Definition  of,  126. 
Voltmeter,  Definition  of,  129. 
,  Description  of,  131. 

w 

Water-Gramme-Degree-Centigrade,  135. 

,  Resistivity  of,  71. 

Rheostat,  327,  328. 

Watt,  Definition  of,  126. 

Waves,  Sinusoidal,  120,  121. 

Weber,  Definition  of,  211. 

Wimshurst  Electrical  Machine,  182,  183. 

Work  and  Activity,  Electric,  124  to  137. 

,  Electric,  Unit  of,  125. 

Rate  of  Doing,  125. 

,  Unit  of  Electrical,  126. 


402  INDEX. 

Y 

Yoke  of  Electromagnet,  203. 

z 

Zinc-Carbon  Cell,  E.  M.  F.  of,  41. 

Voltaic  Cells,  40,  41. 


Elemeitftery  7  :      - 
Electro  -  TechnMal  Series. 


i.D.  and 


Alternating  Electric  Currents,  Electric  Incandescent  Light- 
Electric  Heating,  ing, 

Electromagnetism,  Electric  Motors, 

Electricity  in  Electro-Thera-  Electric  Street  Railways, 

peutics,  Electric  Telephony, 

Electric  Arc  Lighting,  Electric  Telegraphy. 


Cloth,  profusely  illustrated.  Price,  $1.00  per  volume. 


The  above  volumes  have  been  prepared  to  satisfy  a  demand 
which  exists  on  the  part  of  the  general  public  for  reliable  in- 
formation relating  to  the  various  branches  of  electro- technics. 
In  them  will  be  found  concise  and  authoritative  information  con- 
cerning the  several  departments  of  electrical  science  treated, 
and  the  reputation  of  the  authors,  and  their  recognized  ability 
as  writers,  are  a  sufficient  guarantee  as  to  the  accuracy  and 
reliability  of  the  statements.  The  entire  issue,  although  pub- 
lished in  a  series  of  ten  volumes,  is,  nevertheless  so  prepared  that 
each  volume  is  complete  in  itself,  and  can  be  understood  inde- 
pendently of  the  others.  The  books  are  well  printed  on  paper 
of  special  quality,  profusely  illustrated,  and  handsomely  bound 
in  covers  of  a  special  design. 


THE  W.  J.  JOHNSTON  COMPANY,  Publishers, 

253  BROADWAY,  NEW  YORK. 


THIRD  EDITION.     GREA  TL  V  ENLAR  GED< 
A  DICTIONARY  OF 

Electrical  Words,  Terms, 
and  Phrases. 

By  EDWIN  J.  HOUSTON,  Ph.D.  (Princeton). 

AUTHOR  OF 

"Advanced  Primers  of  Electricity";    "Electricity  One 
Hundred  Years  Ago  and  To-day  "  etc.,  etc. 

Cloth,  667  large  octavo  pages,    582    illustrations, 
Price,  $5.00. 

Some  idea  of  the  scope  of  this  important  work  and  of  the  im- 
mense amount  of  labor  involved  in  it,  may  be  formed  when  it  is 
stated  that  it  contains  definitions  of  about  6ooo  distinct  words, 
terms,  or  phrases.  The  dictionary  is  not  a  mere  word-book  ;  the 
words,  terms,  and  phrases  are  invariably  followed  by  a  short,  c©n- 
cise  definition,  giving  the  sense  in  which  they  are  correctly  employed, 
and  a  general  statement  of  the  principles  of  electrical  science  on 
which  the  definition  is  founded.  Each  of  the  great  classes  or  di- 
visions of  electrical  investigation  or  utilization  comes  under  careful 
and  exhaustive  treatment ;  and  while  close  attention  is  given  to  the 
more  settled  and  hackneyed  phraseology  of  the  older  branches  of 
work,  the  newer  words  and  the  novel  departments  they  belong  to 
are  not  less  thoroughly  handled.  Every  source  of  information  has 
been  referred  to,  and  while  libraries  have  been  ransacked,  the  note- 
book of  the  laboratory  and  the  catalogue  of  the  wareroom  have  not 
been  forgotten  or  neglected.  So  far  has  the  work  been  carried  in 
respect  to  the  policy  of  inclusion  that  the  book  has  been  brought 
down  to  date  by  means  of  an  appendix,  in  which  are  placed  the 
very  newest  words,  as  well  as  many  whose  rareness  of  use  had  con- 
signed them  to  obscurity  and  oblivion. 

Copies  of  this  or  any  other  electrical  book  published  will  be  sent  by  mail, 
postage  prepaid,  to  any  address  in  the  worlds  on  receipt  of  price. 


The  W.  J.  Johnston  Company,  Publishers, 

253  BROADWAY,  NEW  YORK. 


AN  tLLUSJUATED  WEEKLY  RECORD  Of  ELECTRIC  RAILWAY  PRACTICE  ANDDEVELOPMENT. 

Established  January  1, 1886. 

THE  DULY  ELECTRIC  RAILWAY  PUBLICATION  IN  THE  WORLD. 


As  the  only  publication  in  the  world  devoted  to  the  electric 
railway  industry,  and  the  only  journal  adequately  treating  the 
numerous  technical  features  involved  in  its  modern  development 
and  practice,  the  Electric  Railway  Gazette  aims  worthily 
to  represent  the  activity  and  progressiveness  of  the  important 
interests  to  which  it  is  devoted. 

Presenting  all  the  news  every  week,  and  describing  current 
improvements  and  developments  immediately  upon  being 
brought  forward,  its  pages  offer  to  those  engaged  in  the  elec- 
tric railway  field  the  timely  advantages  enjoyed  in  other  active 
and  important  branches  of  modern  industry. 


Subscription  in  advance,  One  Year,  $3.00, 

In  the  United  States,  Canada  or  Mexico: 
Foreign  Countries,  $5.00. 


The  W.  J.  Johnston  Company, 

253  BROADWAY,  NEW  YORK. 


THE  MEEK  ELECTRICAL  JOURNAL  Of  AMERICA. 


Read  Wherever  the  English  Language  is  Spoken, 


The  Electrical  World 

is  the  largest,  most  handsomely  illustrated,  and  most  widely 
circulated  electrical  journal  in  the  world. 

It  should  be  read  not  only  by  every  ambitious  electrician 
anxious  to  rise  in  his  profession,  but  by  every  intelligent  Ameri- 
can. 

It  is  noted  for  its  ability,  enterprise,  independence  and 
honesty.  For  thoroughness,  candor  and  progressive  spirit  it 
stands  in  the  foremost  rank  of  special  journalism. 

Always  abreast  of  the  times,  its  treatment  of  everything 
aelating  to  the  practical  and  scientific  development  of  electrical 
knowledge  is  comprehensive  and  authoritative.  Among  its 
many  features  is  a  weekly  Digest  of  Current  Technical  Electri. 
cal  Literature,  which  gives  a  complete  resume  of  current  origi- 
nal contributions  to  electrical  literature  appearing  in  other 
journals  the  world  over. 

Subscription]tocl^fapd°^fyen^,u-si$3  a  Year. 

May  be  ordered  of  any  Newsdealer  at  10  cents  a  week. 


Cloth  Binders  for  THE  ELECTRICAL  "WORLD  postpaid,  $1.00. 


The  W.  J.  Johnston  Company,  Publishers, 

253  BROADWAY,  NEW  YORK. 


AN  INITIAL  FINE  OP  25  CENTS 

WILL  BE  ASSESSED   FOR   FAILURE  TO   RETURN 
THIS   BOOK   ON   THE   DATE  DUE.   THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY    AJUA   TO.    $LOO    Off    THE    SEVENTH    DAY 

ovERmoiogj  Library 

ADD      1                     'J 

)54 

M  i  f\'     ij           iGU /I 

4          !33<+ 

MAY     3  19G3 

M289304 

RMST 

A*^<© 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 

* 


